diff --git a/src/app/api/accounting/accounts/[id]/balance/route.ts b/src/app/api/accounting/accounts/[id]/balance/route.ts
new file mode 100644
index 0000000..139afe1
--- /dev/null
+++ b/src/app/api/accounting/accounts/[id]/balance/route.ts
@@ -0,0 +1,62 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { JournalEntryService } from "@/lib/accounting/journal-entry-service";
+
+/**
+ * GET /api/accounting/accounts/[id]/balance
+ *
+ * Returns the derived balance for an account, computed by summing POSTED journal entry lines.
+ * Balances are NEVER stored — always derived on demand.
+ *
+ * Query params:
+ *   ?asOfDate=ISO8601 — compute balance as of this date (defaults to all-time)
+ *   ?startDate=ISO8601 — include entries on or after this date (defaults to all-time)
+ *
+ * Requires: read on Account (ADMIN or OFFICE_STAFF).
+ *
+ * Response:
+ *   200 OK — { accountId, balance, asOfDate }
+ *   400 Bad Request — no tenant context
+ *   404 Not Found — account not found
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export function GET(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  return withPermission("read", "Account")(
+    async (innerReq: NextRequest, { user }) => {
+      if (!user.tenantId) {
+        return NextResponse.json(
+          { error: "No tenant context — super-admins must use the admin API" },
+          { status: 400 }
+        );
+      }
+
+      const { id } = await params;
+      const { searchParams } = new URL(innerReq.url);
+      const asOfDateParam = searchParams.get("asOfDate");
+      const startDateParam = searchParams.get("startDate");
+
+      const tenantPrisma = withTenantContext(user.tenantId);
+
+      try {
+        const result = await JournalEntryService.getAccountBalance({
+          tenantPrisma,
+          accountId: id,
+          asOfDate: asOfDateParam ? new Date(asOfDateParam) : undefined,
+          startDate: startDateParam ? new Date(startDateParam) : undefined,
+        });
+
+        return NextResponse.json({
+          accountId: result.accountId,
+          balance: result.balance.toFixed(2),
+          asOfDate: result.asOfDate ? result.asOfDate.toISOString() : null,
+        });
+      } catch (err) {
+        const message = err instanceof Error ? err.message : "Failed to get account balance";
+        const status = message.includes("not found") ? 404 : 400;
+        return NextResponse.json({ error: message }, { status });
+      }
+    }
+  )(req);
+}
diff --git a/src/app/api/accounting/journal-entries/[id]/approve/route.ts b/src/app/api/accounting/journal-entries/[id]/approve/route.ts
new file mode 100644
index 0000000..69ffb01
--- /dev/null
+++ b/src/app/api/accounting/journal-entries/[id]/approve/route.ts
@@ -0,0 +1,49 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { JournalEntryService } from "@/lib/accounting/journal-entry-service";
+
+/**
+ * POST /api/accounting/journal-entries/[id]/approve
+ *
+ * Approves a manual journal entry (DRAFT -> POSTED).
+ * Implements maker-checker workflow. Self-approval is allowed for single-person operations.
+ *
+ * Requires: manage on Account (ADMIN only).
+ *
+ * Response:
+ *   200 OK — updated journal entry with lines
+ *   400 Bad Request — entry already posted or invalid state
+ *   404 Not Found — entry not found
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export function POST(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  return withPermission("manage", "Account")(
+    async (_req: NextRequest, { user }) => {
+      if (!user.tenantId) {
+        return NextResponse.json(
+          { error: "No tenant context — super-admins must use the admin API" },
+          { status: 400 }
+        );
+      }
+
+      const { id } = await params;
+      const tenantPrisma = withTenantContext(user.tenantId);
+
+      try {
+        const entry = await JournalEntryService.approveEntry({
+          tenantPrisma,
+          entryId: id,
+          approvedById: user.id,
+        });
+
+        return NextResponse.json(entry);
+      } catch (err) {
+        const message = err instanceof Error ? err.message : "Failed to approve journal entry";
+        const status = message.includes("not found") ? 404 : 400;
+        return NextResponse.json({ error: message }, { status });
+      }
+    }
+  )(req);
+}
diff --git a/src/app/api/accounting/journal-entries/[id]/reverse/route.ts b/src/app/api/accounting/journal-entries/[id]/reverse/route.ts
new file mode 100644
index 0000000..a3d0a21
--- /dev/null
+++ b/src/app/api/accounting/journal-entries/[id]/reverse/route.ts
@@ -0,0 +1,68 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { JournalEntryService } from "@/lib/accounting/journal-entry-service";
+
+/**
+ * POST /api/accounting/journal-entries/[id]/reverse
+ *
+ * Reverses a posted journal entry by creating a new entry with swapped debits/credits.
+ * The original entry is marked REVERSED.
+ *
+ * Body (optional):
+ *   {
+ *     date?: ISO8601 string — accounting date for the reversing entry (defaults to today)
+ *     description?: string — description for the reversing entry
+ *   }
+ *
+ * Requires: manage on Account (ADMIN only).
+ *
+ * Response:
+ *   201 Created — the new reversing entry with lines
+ *   400 Bad Request — entry already reversed or invalid
+ *   404 Not Found — entry not found
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export function POST(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  return withPermission("manage", "Account")(
+    async (innerReq: NextRequest, { user }) => {
+      if (!user.tenantId) {
+        return NextResponse.json(
+          { error: "No tenant context — super-admins must use the admin API" },
+          { status: 400 }
+        );
+      }
+
+      const { id } = await params;
+      const tenantPrisma = withTenantContext(user.tenantId);
+
+      let body: { date?: string; description?: string } = {};
+      try {
+        const text = await innerReq.text();
+        if (text.trim()) {
+          body = JSON.parse(text);
+        }
+      } catch {
+        return NextResponse.json({ error: "Invalid JSON body" }, { status: 400 });
+      }
+
+      try {
+        const reversingEntry = await JournalEntryService.reverseEntry({
+          tenantPrisma,
+          tenantId: user.tenantId,
+          entryId: id,
+          reversedById: user.id,
+          date: body.date ? new Date(body.date) : undefined,
+          description: body.description,
+        });
+
+        return NextResponse.json(reversingEntry, { status: 201 });
+      } catch (err) {
+        const message = err instanceof Error ? err.message : "Failed to reverse journal entry";
+        const status = message.includes("not found") ? 404 : 400;
+        return NextResponse.json({ error: message }, { status });
+      }
+    }
+  )(req);
+}
diff --git a/src/app/api/accounting/journal-entries/[id]/route.ts b/src/app/api/accounting/journal-entries/[id]/route.ts
new file mode 100644
index 0000000..cac4a6e
--- /dev/null
+++ b/src/app/api/accounting/journal-entries/[id]/route.ts
@@ -0,0 +1,48 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+
+/**
+ * GET /api/accounting/journal-entries/[id]
+ *
+ * Returns a single journal entry with its lines.
+ *
+ * Requires: read on Account (ADMIN or OFFICE_STAFF).
+ *
+ * Response:
+ *   200 OK — journal entry with lines
+ *   400 Bad Request — no tenant context
+ *   404 Not Found — entry not found
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export function GET(req: NextRequest, { params }: { params: Promise<{ id: string }> }) {
+  return withPermission("read", "Account")(
+    async (_req: NextRequest, { user }) => {
+      if (!user.tenantId) {
+        return NextResponse.json(
+          { error: "No tenant context — super-admins must use the admin API" },
+          { status: 400 }
+        );
+      }
+
+      const { id } = await params;
+      const tenantPrisma = withTenantContext(user.tenantId);
+
+      const entry = await tenantPrisma.journalEntry.findFirst({
+        where: { id },
+        include: {
+          lines: {
+            orderBy: { createdAt: "asc" },
+          },
+        },
+      });
+
+      if (!entry) {
+        return NextResponse.json({ error: "Journal entry not found" }, { status: 404 });
+      }
+
+      return NextResponse.json(entry);
+    }
+  )(req);
+}
diff --git a/src/app/api/accounting/journal-entries/route.ts b/src/app/api/accounting/journal-entries/route.ts
new file mode 100644
index 0000000..332d2aa
--- /dev/null
+++ b/src/app/api/accounting/journal-entries/route.ts
@@ -0,0 +1,149 @@
+import { NextRequest, NextResponse } from "next/server";
+import { withPermission } from "@/lib/middleware/authorize";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { JournalEntryService } from "@/lib/accounting/journal-entry-service";
+import { JournalEntrySource, JournalEntryStatus } from "@prisma/client";
+
+/**
+ * GET /api/accounting/journal-entries
+ *
+ * Lists journal entries for the authenticated tenant.
+ * Ordered by date descending (most recent first).
+ *
+ * Query params:
+ *   ?startDate=ISO8601 — filter entries on or after this date
+ *   ?endDate=ISO8601   — filter entries on or before this date
+ *   ?status=DRAFT|POSTED|REVERSED|... — filter by status
+ *   ?source=SYSTEM|MANUAL — filter by source
+ *
+ * Requires: read on Account (ADMIN or OFFICE_STAFF).
+ *
+ * Response:
+ *   200 OK — Array of journal entries with lines
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export const GET = withPermission("read", "Account")(
+  async (req: NextRequest, { user }) => {
+    if (!user.tenantId) {
+      return NextResponse.json(
+        { error: "No tenant context — super-admins must use the admin API" },
+        { status: 400 }
+      );
+    }
+
+    const { searchParams } = new URL(req.url);
+    const startDate = searchParams.get("startDate");
+    const endDate = searchParams.get("endDate");
+    const status = searchParams.get("status") as JournalEntryStatus | null;
+    const source = searchParams.get("source") as JournalEntrySource | null;
+
+    // Validate enum values
+    if (status && !Object.values(JournalEntryStatus).includes(status)) {
+      return NextResponse.json({ error: `Invalid status: ${status}` }, { status: 400 });
+    }
+    if (source && !Object.values(JournalEntrySource).includes(source)) {
+      return NextResponse.json({ error: `Invalid source: ${source}` }, { status: 400 });
+    }
+
+    const tenantPrisma = withTenantContext(user.tenantId);
+
+    // Build date filter
+    const dateFilter: Record<string, Date> = {};
+    if (startDate) dateFilter.gte = new Date(startDate);
+    if (endDate) dateFilter.lte = new Date(endDate);
+
+    const entries = await tenantPrisma.journalEntry.findMany({
+      where: {
+        ...(Object.keys(dateFilter).length > 0 ? { date: dateFilter } : {}),
+        ...(status ? { status } : {}),
+        ...(source ? { source } : {}),
+      },
+      include: {
+        lines: {
+          orderBy: { createdAt: "asc" },
+        },
+      },
+      orderBy: { date: "desc" },
+    });
+
+    return NextResponse.json(entries);
+  }
+);
+
+/**
+ * POST /api/accounting/journal-entries
+ *
+ * Creates a manual journal entry. Requires ADMIN role.
+ * Manual entries start with status DRAFT (maker-checker workflow).
+ *
+ * Body:
+ *   {
+ *     date: ISO8601 string,
+ *     description: string,
+ *     lines: [{ accountId: string, debit: number, credit: number, description?: string }]
+ *   }
+ *
+ * Requires: manage on Account (ADMIN only).
+ *
+ * Response:
+ *   201 Created — journal entry with lines
+ *   400 Bad Request — unbalanced entry, missing fields, invalid accountId
+ *   401 Unauthorized — no session
+ *   403 Forbidden — insufficient role
+ */
+export const POST = withPermission("manage", "Account")(
+  async (req: NextRequest, { user }) => {
+    if (!user.tenantId) {
+      return NextResponse.json(
+        { error: "No tenant context — super-admins must use the admin API" },
+        { status: 400 }
+      );
+    }
+
+    let body: {
+      date?: string;
+      description?: string;
+      lines?: Array<{
+        accountId: string;
+        debit: number | string;
+        credit: number | string;
+        description?: string;
+      }>;
+    };
+
+    try {
+      body = await req.json();
+    } catch {
+      return NextResponse.json({ error: "Invalid JSON body" }, { status: 400 });
+    }
+
+    const { date, description, lines } = body;
+
+    if (!date || !description || !lines || !Array.isArray(lines)) {
+      return NextResponse.json(
+        { error: "Missing required fields: date, description, lines" },
+        { status: 400 }
+      );
+    }
+
+    const tenantPrisma = withTenantContext(user.tenantId);
+
+    try {
+      const entry = await JournalEntryService.createEntry({
+        tenantPrisma,
+        tenantId: user.tenantId,
+        date: new Date(date),
+        description,
+        lines,
+        source: JournalEntrySource.MANUAL,
+        createdById: user.id,
+      });
+
+      return NextResponse.json(entry, { status: 201 });
+    } catch (err) {
+      const message = err instanceof Error ? err.message : "Failed to create journal entry";
+      return NextResponse.json({ error: message }, { status: 400 });
+    }
+  }
+);
diff --git a/src/lib/__tests__/journal-entry-service.test.ts b/src/lib/__tests__/journal-entry-service.test.ts
new file mode 100644
index 0000000..46a463c
--- /dev/null
+++ b/src/lib/__tests__/journal-entry-service.test.ts
@@ -0,0 +1,855 @@
+/**
+ * JournalEntryService Integration Tests
+ *
+ * Tests the JournalEntryService — the sole gateway to the accounting ledger.
+ * These tests require a live PostgreSQL database connection.
+ *
+ * WHAT IS TESTED:
+ * - Balanced entry (debit=credit) creates successfully
+ * - Unbalanced entry (debit!=credit) throws error
+ * - Entry with < 2 lines throws error
+ * - Entry with both debit and credit on same line throws error
+ * - Entry in closed period throws error
+ * - Entry in open period succeeds
+ * - JournalEntry has no update/delete methods (immutability by design)
+ * - Reversing entry creates new entry with swapped debits/credits
+ * - Reverse marks original as REVERSED
+ * - Cannot reverse an already-reversed entry
+ * - Reversing entry references original via reversesEntryId
+ * - Account balance computed from entry lines (not stored)
+ * - Balance respects normal balance direction (DEBIT vs CREDIT accounts)
+ * - Balance with asOfDate filters correctly
+ * - Trial balance: total debits = total credits
+ * - Manual entry created with status DRAFT
+ * - System entry created with status POSTED
+ * - Approve changes DRAFT to POSTED
+ * - Self-approve allowed (single-person operation)
+ * - Cannot approve already-POSTED entry
+ * - Entry numbering: JE-{YYYY}-0001 for first, increments correctly
+ *
+ * ISOLATION STRATEGY:
+ * One test tenant created in beforeAll. afterAll cleans up.
+ * Each test uses unique dates/accounts to avoid interference.
+ */
+
+import { prisma } from "@/lib/prisma";
+import { withTenantContext } from "@/lib/prisma-tenant";
+import { JournalEntryService } from "@/lib/accounting/journal-entry-service";
+import { seedChartOfAccounts } from "@/lib/accounting/seed-coa";
+import { closePeriod, getOpenPeriod } from "@/lib/accounting/accounting-period";
+import {
+  JournalEntrySource,
+  JournalEntryStatus,
+  TenantStatus,
+} from "@prisma/client";
+
+// ---------------------------------------------------------------------------
+// Shared test state
+// ---------------------------------------------------------------------------
+
+const TEST_TIMESTAMP = Date.now();
+
+let tenantId: string;
+let tenantPrisma: ReturnType<typeof withTenantContext>;
+
+// Account IDs from seeded COA
+let cashOnHandId: string;   // 1010 — ASSET, DEBIT normal balance
+let cashInBankId: string;   // 1020 — ASSET, DEBIT normal balance
+let arId: string;           // 1100 — ASSET, DEBIT normal balance
+let subCreditsId: string;   // 1150 — ASSET, CREDIT normal balance (contra-asset)
+let subRevenueId: string;   // 4010 — REVENUE, CREDIT normal balance
+
+// A user to act as maker/checker
+let adminUserId: string;
+
+// ---------------------------------------------------------------------------
+// Setup / Teardown
+// ---------------------------------------------------------------------------
+
+beforeAll(async () => {
+  // Create a test tenant
+  const tenant = await prisma.tenant.create({
+    data: {
+      name: `JE Test Tenant ${TEST_TIMESTAMP}`,
+      slug: `je-test-${TEST_TIMESTAMP}`,
+      ownerEmail: `je-owner-${TEST_TIMESTAMP}@test.example`,
+      status: TenantStatus.ACTIVE,
+    },
+  });
+  tenantId = tenant.id;
+  tenantPrisma = withTenantContext(tenantId);
+
+  // Seed Chart of Accounts
+  await prisma.$transaction(async (tx) => {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    await seedChartOfAccounts(tx as any, tenantId);
+  });
+
+  // Look up account IDs we'll use in tests
+  const accounts = await prisma.account.findMany({
+    where: { tenantId, code: { in: ["1010", "1020", "1100", "1150", "4010"] } },
+    select: { id: true, code: true },
+  });
+  const accountMap = new Map(accounts.map((a) => [a.code, a.id]));
+  cashOnHandId = accountMap.get("1010")!;
+  cashInBankId = accountMap.get("1020")!;
+  arId = accountMap.get("1100")!;
+  subCreditsId = accountMap.get("1150")!;
+  subRevenueId = accountMap.get("4010")!;
+
+  // Create an admin user for maker-checker tests
+  const adminUser = await prisma.user.create({
+    data: {
+      email: `je-admin-${TEST_TIMESTAMP}@test.example`,
+      passwordHash: "hashed",
+      firstName: "JE",
+      lastName: "Admin",
+      tenantId,
+      roles: ["ADMIN"],
+      isActive: true,
+    },
+  });
+  adminUserId = adminUser.id;
+});
+
+afterAll(async () => {
+  if (tenantId) {
+    // Clean up in reverse FK dependency order:
+    // 1. Delete all journal entry lines (references journalEntry and account)
+    await prisma.journalEntryLine.deleteMany({ where: { tenantId } }).catch(() => {});
+    // 2. Null out self-referential reversesEntryId to break the FK cycle before deletion
+    await prisma.journalEntry.updateMany({
+      where: { tenantId },
+      data: { reversesEntryId: null },
+    }).catch(() => {});
+    // 3. Delete all journal entries
+    await prisma.journalEntry.deleteMany({ where: { tenantId } }).catch(() => {});
+    // 4. Delete tenant (cascades to users, accounts, accountingPeriods)
+    await prisma.tenant.delete({ where: { id: tenantId } }).catch(() => {});
+  }
+  await prisma.$disconnect();
+});
+
+// ===========================================================================
+// CORE ENFORCEMENT: DEBIT = CREDIT BALANCE
+// ===========================================================================
+
+describe("Balance enforcement", () => {
+  it("creates a balanced entry (debit = credit) successfully", async () => {
+    const entry = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-01-15"),
+      description: "Test: cash to bank transfer",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 100, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 100 },
+      ],
+    });
+
+    expect(entry).toBeDefined();
+    expect(entry.lines).toHaveLength(2);
+    expect(entry.status).toBe(JournalEntryStatus.POSTED);
+  });
+
+  it("rejects an unbalanced entry (debit != credit)", async () => {
+    await expect(
+      JournalEntryService.createEntry({
+        tenantPrisma,
+        tenantId,
+        date: new Date("2026-01-16"),
+        description: "Test: unbalanced",
+        source: JournalEntrySource.SYSTEM,
+        createdById: adminUserId,
+        lines: [
+          { accountId: cashInBankId, debit: 100, credit: 0 },
+          { accountId: cashOnHandId, debit: 0, credit: 50 }, // 100 debit, 50 credit — UNBALANCED
+        ],
+      })
+    ).rejects.toThrow(/unbalanced/i);
+  });
+
+  it("rejects an entry with fewer than 2 lines", async () => {
+    await expect(
+      JournalEntryService.createEntry({
+        tenantPrisma,
+        tenantId,
+        date: new Date("2026-01-17"),
+        description: "Test: single line",
+        source: JournalEntrySource.SYSTEM,
+        createdById: adminUserId,
+        lines: [{ accountId: cashOnHandId, debit: 100, credit: 0 }],
+      })
+    ).rejects.toThrow(/at least 2 lines/i);
+  });
+
+  it("rejects a line with both debit and credit non-zero", async () => {
+    await expect(
+      JournalEntryService.createEntry({
+        tenantPrisma,
+        tenantId,
+        date: new Date("2026-01-18"),
+        description: "Test: both debit and credit",
+        source: JournalEntrySource.SYSTEM,
+        createdById: adminUserId,
+        lines: [
+          { accountId: cashInBankId, debit: 100, credit: 50 }, // INVALID: both non-zero
+          { accountId: cashOnHandId, debit: 0, credit: 50 },
+        ],
+      })
+    ).rejects.toThrow(/cannot have both debit and credit/i);
+  });
+
+  it("rejects a line with both debit and credit as zero", async () => {
+    await expect(
+      JournalEntryService.createEntry({
+        tenantPrisma,
+        tenantId,
+        date: new Date("2026-01-19"),
+        description: "Test: zero line",
+        source: JournalEntrySource.SYSTEM,
+        createdById: adminUserId,
+        lines: [
+          { accountId: cashInBankId, debit: 0, credit: 0 }, // INVALID: both zero
+          { accountId: cashOnHandId, debit: 0, credit: 0 },
+        ],
+      })
+    ).rejects.toThrow(/must have either a debit or credit/i);
+  });
+
+  it("handles decimal amounts correctly (e.g., 100.50)", async () => {
+    const entry = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-01-20"),
+      description: "Test: decimal amounts",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: arId, debit: "100.50", credit: 0 },
+        { accountId: subRevenueId, debit: 0, credit: "100.50" },
+      ],
+    });
+
+    // Prisma Decimal.toString() may omit trailing zeros; use toNumber() for comparison
+    expect(entry.lines[0].debit.toNumber()).toBe(100.50);
+    expect(entry.lines[1].credit.toNumber()).toBe(100.50);
+  });
+});
+
+// ===========================================================================
+// CLOSED PERIOD PROTECTION
+// ===========================================================================
+
+describe("Closed period protection", () => {
+  const CLOSED_YEAR = 2025;
+  const CLOSED_MONTH = 1; // January 2025
+
+  beforeAll(async () => {
+    // Create and then close January 2025 for this tenant
+    const period = await getOpenPeriod(prisma, tenantId, CLOSED_YEAR, CLOSED_MONTH);
+    await closePeriod(prisma, period.id, adminUserId);
+  });
+
+  it("blocks posting to a closed period", async () => {
+    await expect(
+      JournalEntryService.createEntry({
+        tenantPrisma,
+        tenantId,
+        date: new Date("2025-01-15"), // January 2025 is CLOSED
+        description: "Test: entry in closed period",
+        source: JournalEntrySource.SYSTEM,
+        createdById: adminUserId,
+        lines: [
+          { accountId: cashInBankId, debit: 100, credit: 0 },
+          { accountId: cashOnHandId, debit: 0, credit: 100 },
+        ],
+      })
+    ).rejects.toThrow(/closed/i);
+  });
+
+  it("allows posting to an open period", async () => {
+    const entry = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-02-15"), // February 2026 is OPEN
+      description: "Test: entry in open period",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 200, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 200 },
+      ],
+    });
+
+    expect(entry).toBeDefined();
+    expect(entry.status).toBe(JournalEntryStatus.POSTED);
+  });
+});
+
+// ===========================================================================
+// IMMUTABILITY
+// ===========================================================================
+
+describe("Immutability", () => {
+  it("JournalEntryService has no update method", () => {
+    // Verify at the runtime level that no update method exists on the service
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    expect(typeof (JournalEntryService as any).updateEntry).toBe("undefined");
+  });
+
+  it("JournalEntryService has no delete method", () => {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    expect(typeof (JournalEntryService as any).deleteEntry).toBe("undefined");
+  });
+});
+
+// ===========================================================================
+// REVERSING ENTRIES
+// ===========================================================================
+
+describe("Reversing entries", () => {
+  let originalEntryId: string;
+  let originalEntryNumber: string;
+
+  beforeAll(async () => {
+    // Create an entry to reverse
+    const entry = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-03-01"),
+      description: "Original entry for reversal test",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: arId, debit: 500, credit: 0 },
+        { accountId: subRevenueId, debit: 0, credit: 500 },
+      ],
+    });
+    originalEntryId = entry.id;
+    originalEntryNumber = entry.entryNumber;
+  });
+
+  it("creates a reversing entry with swapped debits and credits", async () => {
+    const reversingEntry = await JournalEntryService.reverseEntry({
+      tenantPrisma,
+      tenantId,
+      entryId: originalEntryId,
+      reversedById: adminUserId,
+      date: new Date("2026-03-02"),
+    });
+
+    expect(reversingEntry).toBeDefined();
+    expect(reversingEntry.status).toBe(JournalEntryStatus.POSTED);
+    expect(reversingEntry.source).toBe(JournalEntrySource.SYSTEM);
+
+    // Lines should be swapped: AR was debit 500, now should be credit 500
+    const arLine = reversingEntry.lines.find(
+      (l: { accountId: string }) => l.accountId === arId
+    );
+    const revLine = reversingEntry.lines.find(
+      (l: { accountId: string }) => l.accountId === subRevenueId
+    );
+    // Use toNumber() for comparison — Prisma Decimal.toString() may omit trailing zeros
+    expect(arLine?.credit.toNumber()).toBe(500);
+    expect(arLine?.debit.toNumber()).toBe(0);
+    expect(revLine?.debit.toNumber()).toBe(500);
+    expect(revLine?.credit.toNumber()).toBe(0);
+  });
+
+  it("marks the original entry as REVERSED", async () => {
+    const original = await tenantPrisma.journalEntry.findFirst({
+      where: { id: originalEntryId },
+    });
+    expect(original?.status).toBe(JournalEntryStatus.REVERSED);
+  });
+
+  it("reversing entry references original via reversesEntryId", async () => {
+    const reversingEntry = await tenantPrisma.journalEntry.findFirst({
+      where: { reversesEntryId: originalEntryId },
+    });
+    expect(reversingEntry).not.toBeNull();
+    expect(reversingEntry?.reversesEntryId).toBe(originalEntryId);
+  });
+
+  it("cannot reverse an already-reversed entry", async () => {
+    await expect(
+      JournalEntryService.reverseEntry({
+        tenantPrisma,
+        tenantId,
+        entryId: originalEntryId,
+        reversedById: adminUserId,
+      })
+    ).rejects.toThrow(/already been reversed/i);
+  });
+
+  it("reverse with custom description and date", async () => {
+    // Create another entry to reverse
+    const entry = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-03-05"),
+      description: "Entry to reverse with custom description",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashOnHandId, debit: 300, credit: 0 },
+        { accountId: cashInBankId, debit: 0, credit: 300 },
+      ],
+    });
+
+    const reversal = await JournalEntryService.reverseEntry({
+      tenantPrisma,
+      tenantId,
+      entryId: entry.id,
+      reversedById: adminUserId,
+      date: new Date("2026-03-10"),
+      description: "Custom reversal description",
+    });
+
+    expect(reversal.description).toBe("Custom reversal description");
+    expect(reversal.date.toISOString().slice(0, 10)).toBe("2026-03-10");
+    // Entry number should be for 2026
+    expect(reversal.entryNumber).toMatch(/^JE-2026-/);
+  });
+
+  it("reversal description defaults to Reversal of {entryNumber}: {description}", async () => {
+    const entry = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-03-06"),
+      description: "Default reversal desc test",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashOnHandId, debit: 150, credit: 0 },
+        { accountId: cashInBankId, debit: 0, credit: 150 },
+      ],
+    });
+
+    const reversal = await JournalEntryService.reverseEntry({
+      tenantPrisma,
+      tenantId,
+      entryId: entry.id,
+      reversedById: adminUserId,
+    });
+
+    expect(reversal.description).toContain(entry.entryNumber);
+    expect(reversal.description).toContain("Default reversal desc test");
+  });
+});
+
+// ===========================================================================
+// ACCOUNT BALANCE DERIVATION
+// ===========================================================================
+
+describe("Account balance derivation", () => {
+  // Use a unique far-future year to isolate balance tests from all others
+  // No other test in this file uses year 2097, so balances are precise
+  const BALANCE_YEAR = 2097;
+
+  beforeAll(async () => {
+    // Entry 1: AR debit 1000, Revenue credit 1000 (April 1)
+    await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date(`${BALANCE_YEAR}-04-01`),
+      description: "Balance test: invoice",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: arId, debit: 1000, credit: 0, description: "AR debit" },
+        { accountId: subRevenueId, debit: 0, credit: 1000, description: "Revenue credit" },
+      ],
+    });
+
+    // Entry 2: AR debit 500, Revenue credit 500 (April 5)
+    await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date(`${BALANCE_YEAR}-04-05`),
+      description: "Balance test: second invoice",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: arId, debit: 500, credit: 0 },
+        { accountId: subRevenueId, debit: 0, credit: 500 },
+      ],
+    });
+
+    // Entry 3: Cash collected — AR credit 800, Cash debit 800 (April 10)
+    await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date(`${BALANCE_YEAR}-04-10`),
+      description: "Balance test: payment received",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashOnHandId, debit: 800, credit: 0 },
+        { accountId: arId, debit: 0, credit: 800 },
+      ],
+    });
+  });
+
+  it("computes account balance from POSTED journal entry lines (not stored)", async () => {
+    // AR balance for BALANCE_YEAR only: 1000 + 500 - 800 = 700
+    // Use startDate + asOfDate to scope to BALANCE_YEAR only
+    const { balance } = await JournalEntryService.getAccountBalance({
+      tenantPrisma,
+      accountId: arId,
+      startDate: new Date(`${BALANCE_YEAR}-01-01`),
+      asOfDate: new Date(`${BALANCE_YEAR}-12-31`),
+    });
+    expect(balance.toNumber()).toBe(700);
+  });
+
+  it("balance respects DEBIT normal balance (ASSET account)", async () => {
+    // Cash on Hand: DEBIT normal balance — balance = sum(debit) - sum(credit)
+    // Only entry 3 touches Cash on Hand in BALANCE_YEAR: debit 800
+    const { balance } = await JournalEntryService.getAccountBalance({
+      tenantPrisma,
+      accountId: cashOnHandId,
+      startDate: new Date(`${BALANCE_YEAR}-01-01`),
+      asOfDate: new Date(`${BALANCE_YEAR}-12-31`),
+    });
+    expect(balance.toNumber()).toBe(800);
+  });
+
+  it("balance respects CREDIT normal balance (REVENUE account)", async () => {
+    // Subscription Revenue: CREDIT normal balance — balance = sum(credit) - sum(debit)
+    // Revenue credited 1000 + 500 = 1500 in BALANCE_YEAR
+    const { balance } = await JournalEntryService.getAccountBalance({
+      tenantPrisma,
+      accountId: subRevenueId,
+      startDate: new Date(`${BALANCE_YEAR}-01-01`),
+      asOfDate: new Date(`${BALANCE_YEAR}-12-31`),
+    });
+    expect(balance.toNumber()).toBe(1500);
+  });
+
+  it("balance with asOfDate filters correctly (excludes future entries)", async () => {
+    // All queries scoped to BALANCE_YEAR only (using startDate to exclude prior entries)
+    // As of April 3: only the April 1 entry included → AR = 1000
+    const { balance: balanceApril3 } = await JournalEntryService.getAccountBalance({
+      tenantPrisma,
+      accountId: arId,
+      startDate: new Date(`${BALANCE_YEAR}-01-01`),
+      asOfDate: new Date(`${BALANCE_YEAR}-04-03`),
+    });
+
+    // As of April 6: April 1 + April 5 entries included → AR = 1000 + 500 = 1500
+    const { balance: balanceApril6 } = await JournalEntryService.getAccountBalance({
+      tenantPrisma,
+      accountId: arId,
+      startDate: new Date(`${BALANCE_YEAR}-01-01`),
+      asOfDate: new Date(`${BALANCE_YEAR}-04-06`),
+    });
+
+    // As of April 30: all three entries included → AR = 1000 + 500 - 800 = 700
+    const { balance: balanceApril30 } = await JournalEntryService.getAccountBalance({
+      tenantPrisma,
+      accountId: arId,
+      startDate: new Date(`${BALANCE_YEAR}-01-01`),
+      asOfDate: new Date(`${BALANCE_YEAR}-04-30`),
+    });
+
+    expect(balanceApril3.toNumber()).toBe(1000);
+    expect(balanceApril6.toNumber()).toBe(1500);
+    expect(balanceApril30.toNumber()).toBe(700);
+  });
+
+  it("balance returns 0 for account with no entries in the isolated period", async () => {
+    // Cash in Bank had no entries in BALANCE_YEAR
+    const { balance } = await JournalEntryService.getAccountBalance({
+      tenantPrisma,
+      accountId: cashInBankId,
+      startDate: new Date(`${BALANCE_YEAR}-01-01`),
+      asOfDate: new Date(`${BALANCE_YEAR}-12-31`),
+    });
+    expect(balance.toNumber()).toBe(0);
+  });
+
+  it("throws for non-existent accountId", async () => {
+    await expect(
+      JournalEntryService.getAccountBalance({
+        tenantPrisma,
+        accountId: "non-existent-account-id",
+      })
+    ).rejects.toThrow(/not found/i);
+  });
+});
+
+// ===========================================================================
+// TRIAL BALANCE
+// ===========================================================================
+
+describe("Trial balance", () => {
+  it("returns trial balance with total debits equal to total credits", async () => {
+    const trialBalance = await JournalEntryService.getTrialBalance({ tenantPrisma });
+
+    const totalDebits = trialBalance.reduce(
+      (sum, line) => sum + line.debitBalance.toNumber(),
+      0
+    );
+    const totalCredits = trialBalance.reduce(
+      (sum, line) => sum + line.creditBalance.toNumber(),
+      0
+    );
+
+    // Round to 2 decimal places to avoid float comparison issues
+    expect(Math.round(totalDebits * 100)).toBe(Math.round(totalCredits * 100));
+  });
+
+  it("includes all accounts in the COA (even those with zero balance)", async () => {
+    const trialBalance = await JournalEntryService.getTrialBalance({ tenantPrisma });
+    // The ISP COA has 28 accounts; trial balance should include all
+    expect(trialBalance.length).toBeGreaterThanOrEqual(20);
+  });
+
+  it("trial balance contains account codes and names", async () => {
+    const trialBalance = await JournalEntryService.getTrialBalance({ tenantPrisma });
+    const arLine = trialBalance.find((l) => l.accountCode === "1100");
+    expect(arLine).toBeDefined();
+    expect(arLine?.accountName).toBe("Accounts Receivable");
+  });
+
+  it("trial balance with asOfDate filters correctly", async () => {
+    // Trial balance in the far past should show zero balances (no entries before 2000)
+    const trialBalance2000 = await JournalEntryService.getTrialBalance({
+      tenantPrisma,
+      asOfDate: new Date("2000-01-01"),
+    });
+    const total = trialBalance2000.reduce(
+      (sum, l) => sum + l.debitBalance.toNumber() + l.creditBalance.toNumber(),
+      0
+    );
+    expect(total).toBe(0);
+  });
+});
+
+// ===========================================================================
+// MAKER-CHECKER WORKFLOW
+// ===========================================================================
+
+describe("Maker-checker workflow", () => {
+  it("MANUAL source creates entry with DRAFT status", async () => {
+    const entry = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-05-01"),
+      description: "Manual: cash on hand to bank",
+      source: JournalEntrySource.MANUAL,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 250, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 250 },
+      ],
+    });
+
+    expect(entry.status).toBe(JournalEntryStatus.DRAFT);
+    expect(entry.source).toBe(JournalEntrySource.MANUAL);
+    expect(entry.approvedById).toBeNull();
+  });
+
+  it("SYSTEM source creates entry with POSTED status", async () => {
+    const entry = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-05-02"),
+      description: "System: auto-generated entry",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 100, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 100 },
+      ],
+    });
+
+    expect(entry.status).toBe(JournalEntryStatus.POSTED);
+    expect(entry.source).toBe(JournalEntrySource.SYSTEM);
+  });
+
+  it("approve changes DRAFT to POSTED and sets approvedById and approvedAt", async () => {
+    const draft = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-05-03"),
+      description: "Manual entry for approval test",
+      source: JournalEntrySource.MANUAL,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 400, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 400 },
+      ],
+    });
+
+    expect(draft.status).toBe(JournalEntryStatus.DRAFT);
+
+    const approved = await JournalEntryService.approveEntry({
+      tenantPrisma,
+      entryId: draft.id,
+      approvedById: adminUserId,
+    });
+
+    expect(approved.status).toBe(JournalEntryStatus.POSTED);
+    expect(approved.approvedById).toBe(adminUserId);
+    expect(approved.approvedAt).not.toBeNull();
+  });
+
+  it("self-approve is allowed (same user creates and approves)", async () => {
+    const draft = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-05-04"),
+      description: "Self-approve test",
+      source: JournalEntrySource.MANUAL,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 75, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 75 },
+      ],
+    });
+
+    // Same user approves their own entry — should succeed
+    const approved = await JournalEntryService.approveEntry({
+      tenantPrisma,
+      entryId: draft.id,
+      approvedById: adminUserId, // same as createdById
+    });
+
+    expect(approved.status).toBe(JournalEntryStatus.POSTED);
+    expect(approved.createdById).toBe(approved.approvedById);
+  });
+
+  it("cannot approve an already-POSTED entry", async () => {
+    // SYSTEM entry is already POSTED
+    const posted = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date("2026-05-05"),
+      description: "Already posted",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 50, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 50 },
+      ],
+    });
+
+    await expect(
+      JournalEntryService.approveEntry({
+        tenantPrisma,
+        entryId: posted.id,
+        approvedById: adminUserId,
+      })
+    ).rejects.toThrow(/cannot approve/i);
+  });
+
+  it("cannot approve a non-existent entry", async () => {
+    await expect(
+      JournalEntryService.approveEntry({
+        tenantPrisma,
+        entryId: "non-existent-id",
+        approvedById: adminUserId,
+      })
+    ).rejects.toThrow(/not found/i);
+  });
+});
+
+// ===========================================================================
+// ENTRY NUMBERING
+// ===========================================================================
+
+describe("Entry numbering", () => {
+  it("first entry of year gets JE-{YYYY}-0001 format", async () => {
+    // Use a unique future year to avoid interference with other tests
+    const UNIQUE_YEAR = 2099;
+
+    const entry = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date(`${UNIQUE_YEAR}-01-01`),
+      description: "First entry of year test",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 10, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 10 },
+      ],
+    });
+
+    expect(entry.entryNumber).toBe(`JE-${UNIQUE_YEAR}-0001`);
+  });
+
+  it("subsequent entries for the same year increment correctly", async () => {
+    // Use 2098 year to avoid interference
+    const UNIQUE_YEAR = 2098;
+
+    const entry1 = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date(`${UNIQUE_YEAR}-01-01`),
+      description: "First entry",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 10, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 10 },
+      ],
+    });
+
+    const entry2 = await JournalEntryService.createEntry({
+      tenantPrisma,
+      tenantId,
+      date: new Date(`${UNIQUE_YEAR}-02-01`),
+      description: "Second entry",
+      source: JournalEntrySource.SYSTEM,
+      createdById: adminUserId,
+      lines: [
+        { accountId: cashInBankId, debit: 20, credit: 0 },
+        { accountId: cashOnHandId, debit: 0, credit: 20 },
+      ],
+    });
+
+    expect(entry1.entryNumber).toBe(`JE-${UNIQUE_YEAR}-0001`);
+    expect(entry2.entryNumber).toBe(`JE-${UNIQUE_YEAR}-0002`);
+  });
+
+  it("entry numbers are unique per tenant per year", async () => {
+    // All 2026 entries should have unique numbers
+    const entries = await tenantPrisma.journalEntry.findMany({
+      where: { entryNumber: { startsWith: "JE-2026-" } },
+      select: { entryNumber: true },
+    });
+
+    const numbers = entries.map((e: { entryNumber: string }) => e.entryNumber);
+    const uniqueNumbers = new Set(numbers);
+    expect(uniqueNumbers.size).toBe(numbers.length);
+  });
+});
+
+// ===========================================================================
+// ACCOUNT VALIDATION
+// ===========================================================================
+
+describe("Account validation", () => {
+  it("rejects entry with non-existent accountId", async () => {
+    await expect(
+      JournalEntryService.createEntry({
+        tenantPrisma,
+        tenantId,
+        date: new Date("2026-06-01"),
+        description: "Invalid account test",
+        source: JournalEntrySource.SYSTEM,
+        createdById: adminUserId,
+        lines: [
+          { accountId: "non-existent-account-id", debit: 100, credit: 0 },
+          { accountId: cashOnHandId, debit: 0, credit: 100 },
+        ],
+      })
+    ).rejects.toThrow(/not found|do not belong/i);
+  });
+});
diff --git a/src/lib/accounting/journal-entry-service.ts b/src/lib/accounting/journal-entry-service.ts
index 1322fd5..233e0df 100644
--- a/src/lib/accounting/journal-entry-service.ts
+++ b/src/lib/accounting/journal-entry-service.ts
@@ -81,7 +81,10 @@ export interface ReverseEntryInput {
 export interface GetAccountBalanceInput {
   tenantPrisma: TenantPrismaClient;
   accountId: string;
+  /** Include only entries on or before this date (inclusive) */
   asOfDate?: Date;
+  /** Include only entries on or after this date (inclusive) */
+  startDate?: Date;
 }
 
 export interface AccountBalanceResult {
@@ -442,7 +445,7 @@ export class JournalEntryService {
    * @param asOfDate - If provided, only includes entries dated on or before this date
    */
   static async getAccountBalance(input: GetAccountBalanceInput): Promise<AccountBalanceResult> {
-    const { tenantPrisma, accountId, asOfDate } = input;
+    const { tenantPrisma, accountId, asOfDate, startDate } = input;
 
     // Fetch account to determine normal balance direction
     const account = await tenantPrisma.account.findFirst({
@@ -454,8 +457,11 @@ export class JournalEntryService {
       throw new Error(`Account not found: ${accountId}`);
     }
 
-    // Build date filter for asOfDate
-    const dateFilter = asOfDate ? { date: { lte: asOfDate } } : {};
+    // Build date filter for date range
+    const dateConditions: Record<string, Date> = {};
+    if (asOfDate) dateConditions.lte = asOfDate;
+    if (startDate) dateConditions.gte = startDate;
+    const dateFilter = Object.keys(dateConditions).length > 0 ? { date: dateConditions } : {};
 
     // Aggregate debit and credit sums from POSTED journal entry lines
     const result = await tenantPrisma.journalEntryLine.aggregate({