diff --git a/src/engine/reports.ts b/src/engine/reports.ts
index f81abc1..cec4437 100644
--- a/src/engine/reports.ts
+++ b/src/engine/reports.ts
@@ -45,11 +45,20 @@ export async function getProfitAndLoss(opts: {
   from_date?: string;
   to_date?: string;
 }): Promise<ProfitAndLossResult> {
+  // When a date range is provided, the transactions.date filters below
+  // select across periods — so we must NOT also restrict to a single
+  // transaction_lines.period_id, which would neutralise the date range.
+  // When no date range is given, fall back to the single-period filter
+  // so callers still get "just this period" behaviour by default.
   let q = db('transaction_lines')
     .join('accounts', 'transaction_lines.account_code', 'accounts.code')
     .join('transactions', 'transaction_lines.transaction_id', 'transactions.transaction_id')
     .whereIn('accounts.type', ['REVENUE', 'EXPENSE'])
-    .where('transaction_lines.period_id', opts.period_id)
+    .modify((qb) => {
+      if (!opts.from_date && !opts.to_date) {
+        qb.where('transaction_lines.period_id', opts.period_id);
+      }
+    })
     .select(
       'accounts.code',
       'accounts.name',
diff --git a/tests/integration/reports.test.ts b/tests/integration/reports.test.ts
index f2f2987..07f7ffa 100644
--- a/tests/integration/reports.test.ts
+++ b/tests/integration/reports.test.ts
@@ -165,6 +165,81 @@ describe('getProfitAndLoss', () => {
   });
 });
 
+// ---------------------------------------------------------------------------
+// Profit and Loss report — date range spanning multiple periods (Bug 12)
+// ---------------------------------------------------------------------------
+
+describe('getProfitAndLoss — date range across multiple periods', () => {
+  let periodA: string;
+  let periodB: string;
+
+  beforeAll(async () => {
+    periodA = uniquePeriod();
+    periodB = uniquePeriod();
+    await createPeriod(periodA);
+    await createPeriod(periodB);
+
+    // Period A: CUSTOMER_INVOICE £1,200 gross → £1,000 revenue + £200 VAT
+    await post({
+      transaction_type: 'CUSTOMER_INVOICE',
+      date: `${periodA}-10`,
+      period_id: periodA,
+      amount: 1200,
+      idempotency_key: `pl-range-a-${periodA}`,
+    });
+
+    // Period B: CUSTOMER_INVOICE £2,400 gross → £2,000 revenue + £400 VAT
+    await post({
+      transaction_type: 'CUSTOMER_INVOICE',
+      date: `${periodB}-10`,
+      period_id: periodB,
+      amount: 2400,
+      idempotency_key: `pl-range-b-${periodB}`,
+    });
+  });
+
+  afterAll(async () => {
+    await deletePeriod(periodA);
+    await deletePeriod(periodB);
+  });
+
+  it('no date range: returns only the single period specified in period_id', async () => {
+    const report = await getProfitAndLoss({ period_id: periodA });
+    expect(report.total_revenue).toBe('1000.00');
+  });
+
+  it('date range spanning both periods: aggregates revenue across them', async () => {
+    const report = await getProfitAndLoss({
+      period_id: periodA,
+      from_date: `${periodA}-01`,
+      to_date: `${periodB}-28`,
+    });
+    // £1,000 (period A) + £2,000 (period B) = £3,000
+    expect(report.total_revenue).toBe('3000.00');
+  });
+
+  it('date range confined to period B: returns only period B revenue (period_id is ignored when dates are set)', async () => {
+    const report = await getProfitAndLoss({
+      period_id: periodA,
+      from_date: `${periodB}-01`,
+      to_date: `${periodB}-28`,
+    });
+    expect(report.total_revenue).toBe('2000.00');
+  });
+
+  it('aggregate across range equals sum of individual monthly reports', async () => {
+    const a = await getProfitAndLoss({ period_id: periodA });
+    const b = await getProfitAndLoss({ period_id: periodB });
+    const combined = await getProfitAndLoss({
+      period_id: periodA,
+      from_date: `${periodA}-01`,
+      to_date: `${periodB}-28`,
+    });
+    const expected = new Decimal(a.total_revenue).plus(b.total_revenue).toFixed(2);
+    expect(combined.total_revenue).toBe(expected);
+  });
+});
+
 // ---------------------------------------------------------------------------
 // Balance Sheet report
 // ---------------------------------------------------------------------------