diff --git a/CHANGES.md b/CHANGES.md
index 7539f2e..010b6d9 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased
+- Billing: CSV export of a tenant's invoices (`GET /billing/invoices.csv`), with optional `from`, `to` and `currency` filters.
 
 ## 3.2.0
 - Returns: partial refunds.
diff --git a/src/lib/csv.js b/src/lib/csv.js
new file mode 100644
index 0000000..6aeb0e5
--- /dev/null
+++ b/src/lib/csv.js
@@ -0,0 +1,9 @@
+// RFC 4180 field quoting.
+export function csvField(v) {
+  const s = String(v ?? '');
+  return /[",\r\n]/.test(s) ? `"${s.replace(/"/g, '""')}"` : s;
+}
+
+export function toCsv(header, rows) {
+  return [header, ...rows].map((r) => r.map(csvField).join(',')).join('\n') + '\n';
+}
diff --git a/src/modules/billing/formatters.js b/src/modules/billing/formatters.js
index f64ce28..796fd8b 100644
--- a/src/modules/billing/formatters.js
+++ b/src/modules/billing/formatters.js
@@ -1,5 +1,16 @@
 import { formatCents } from '../../lib/money.js';
+import { isoDate } from '../../lib/time.js';
 
 export function formatInvoice(inv) {
   return { ...inv, total: formatCents(inv.total) };
 }
+
+export const INVOICE_CSV_COLUMNS = ['id', 'date', 'customer', 'subtotal', 'discount', 'tax', 'total', 'currency'];
+
+export function invoiceCsvRow(inv) {
+  return [
+    inv.id, isoDate(inv.date), inv.customer,
+    formatCents(inv.subtotal), formatCents(inv.discount), formatCents(inv.tax), formatCents(inv.total),
+    inv.currency,
+  ];
+}
diff --git a/src/modules/billing/routes.js b/src/modules/billing/routes.js
index 0ac3d5c..8b585fe 100644
--- a/src/modules/billing/routes.js
+++ b/src/modules/billing/routes.js
@@ -1,8 +1,14 @@
-import { ok, wrap } from '../../lib/http.js';
-import { getInvoice, listInvoices } from './service.js';
-import { formatInvoice } from './formatters.js';
+import { ok, text, wrap } from '../../lib/http.js';
+import { toCsv } from '../../lib/csv.js';
+import { exportInvoices, getInvoice, listInvoices } from './service.js';
+import { INVOICE_CSV_COLUMNS, formatInvoice, invoiceCsvRow } from './formatters.js';
+import { validateExportQuery } from './validators.js';
 
 export const routes = [
   ['GET', '/billing/invoices', wrap(async (req) => ok(listInvoices(req.query).map(formatInvoice)))],
+  ['GET', '/billing/invoices.csv', wrap(async (req) => {
+    const rows = exportInvoices(validateExportQuery(req.query)).map(invoiceCsvRow);
+    return text(toCsv(INVOICE_CSV_COLUMNS, rows), 'text/csv');
+  })],
   ['GET', '/billing/invoices/:id', wrap(async (req) => ok(formatInvoice(getInvoice(req.params.id))))],
 ];
diff --git a/src/modules/billing/service.js b/src/modules/billing/service.js
index d84492f..760b896 100644
--- a/src/modules/billing/service.js
+++ b/src/modules/billing/service.js
@@ -1,4 +1,5 @@
 import { NotFoundError } from '../../lib/errors.js';
+import { inRange } from '../../lib/time.js';
 import { discountFor } from './discounts.js';
 import { taxFor } from './tax.js';
 import { TAX_REGION } from './constants.js';
@@ -21,3 +22,13 @@ export function getInvoice(id) {
 export function listInvoices({ tenantId } = {}) {
   return allInvoices().filter((i) => !tenantId || i.tenantId === tenantId);
 }
+
+// Invoices for the CSV export: 404 if the tenant has none at all, then date-filtered and sorted.
+export function exportInvoices({ tenantId, from, to, currency }) {
+  const all = listInvoices({ tenantId });
+  if (all.length === 0) throw new NotFoundError('tenant ' + tenantId);
+  const cmp = (a, b) => (a < b ? -1 : a > b ? 1 : 0);
+  return all
+    .filter((i) => inRange(i.date, from, to) && (!currency || i.currency === currency))
+    .sort((a, b) => cmp(a.date, b.date) || cmp(a.id, b.id));
+}
diff --git a/src/modules/billing/validators.js b/src/modules/billing/validators.js
index 501b848..117bb2d 100644
--- a/src/modules/billing/validators.js
+++ b/src/modules/billing/validators.js
@@ -1,6 +1,24 @@
-import { oneOf } from '../../lib/validate.js';
+import { oneOf, requireString } from '../../lib/validate.js';
+import { ValidationError } from '../../lib/errors.js';
+import { isoDate } from '../../lib/time.js';
 import { CURRENCIES } from './constants.js';
 
+function optionalDate(query, key) {
+  const v = query?.[key];
+  if (v === undefined || v === '') return undefined;
+  if (!/^\d{4}-\d{2}-\d{2}$/.test(v) || Number.isNaN(Date.parse(v)) || isoDate(v) !== v) throw new ValidationError(`${key} must be YYYY-MM-DD`);
+  return v;
+}
+
+export function validateExportQuery(query) {
+  return {
+    tenantId: requireString(query, 'tenant'),
+    from: optionalDate(query, 'from'),
+    to: optionalDate(query, 'to'),
+    currency: query?.currency === undefined ? undefined : validateCurrency(query),
+  };
+}
+
 export function validateCurrency(input) {
   return oneOf(input, 'currency', CURRENCIES);
 }
diff --git a/tests/lib.test.js b/tests/lib.test.js
index a03e86e..e484b18 100644
--- a/tests/lib.test.js
+++ b/tests/lib.test.js
@@ -2,6 +2,16 @@ import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import { formatCents, percentOf } from '../src/lib/money.js';
 import { inRange, isoDate } from '../src/lib/time.js';
+import { csvField, toCsv } from '../src/lib/csv.js';
+
+test('lib: csv', () => {
+  assert.equal(csvField('plain'), 'plain');
+  assert.equal(csvField('a,b'), '"a,b"');
+  assert.equal(csvField('say "hi"'), '"say ""hi"""');
+  assert.equal(csvField('a\nb'), '"a\nb"');
+  assert.equal(toCsv(['a', 'b'], []), 'a,b\n');
+  assert.equal(toCsv(['a', 'b'], [[1, 'x,y']]), 'a,b\n1,"x,y"\n');
+});
 
 test('lib: money', () => {
   assert.equal(formatCents(105), '1.05');
diff --git a/tests/server.test.js b/tests/server.test.js
index e836eaa..ec577d4 100644
--- a/tests/server.test.js
+++ b/tests/server.test.js
@@ -17,3 +17,61 @@ test('server: invoice route and 404s', async () => {
   assert.equal((await app.handle('GET', '/billing/invoices/nope')).status, 404);
   assert.equal((await app.handle('GET', '/nothing')).status, 404);
 });
+
+const HEADER = 'id,date,customer,subtotal,discount,tax,total,currency';
+
+test('server: invoices.csv full export', async () => {
+  const app = createApp();
+  const r = await app.handle('GET', '/billing/invoices.csv?tenant=acme');
+  assert.equal(r.status, 200);
+  assert.equal(r.headers['content-type'], 'text/csv');
+  assert.equal(r.body, [
+    HEADER,
+    'inv_0006,2026-08-31,Blue Fern Ltd,30.00,0.00,2.10,32.10,USD',
+    'inv_0001,2026-09-02,Blue Fern Ltd,120.00,12.00,7.56,115.56,USD',
+    'inv_0002,2026-09-15,"Smith, Jones & Co",49.99,0.00,3.50,53.49,USD',
+    'inv_0003,2026-09-15,"The ""Quick"" Shop",200.00,5.00,39.00,234.00,EUR',
+    'inv_0004,2026-10-01,Ortega Freight,1.05,0.00,0.07,1.12,USD',
+    '',
+  ].join('\n'));
+});
+
+test('server: invoices.csv date range is inclusive', async () => {
+  const app = createApp();
+  const ids = async (qs) => (await app.handle('GET', '/billing/invoices.csv?tenant=acme' + qs)).body
+    .split('\n').slice(1, -1).map((l) => l.slice(0, 8));
+  assert.deepEqual(await ids('&from=2026-09-15&to=2026-09-15'), ['inv_0002', 'inv_0003']);
+  assert.deepEqual(await ids('&from=2026-10-01'), ['inv_0004']);
+  assert.deepEqual(await ids('&to=2026-08-31'), ['inv_0006']);
+});
+
+test('server: invoices.csv header only when nothing matches', async () => {
+  const app = createApp();
+  const r = await app.handle('GET', '/billing/invoices.csv?tenant=acme&from=2027-01-01');
+  assert.equal(r.status, 200);
+  assert.equal(r.body, HEADER + '\n');
+});
+
+test('server: invoices.csv currency filter', async () => {
+  const app = createApp();
+  const ids = async (qs) => (await app.handle('GET', '/billing/invoices.csv?tenant=acme' + qs)).body
+    .split('\n').slice(1, -1).map((l) => l.slice(0, 8));
+  assert.deepEqual(await ids('&currency=EUR'), ['inv_0003']);
+  assert.deepEqual(await ids('&currency=USD&from=2026-09-15'), ['inv_0002', 'inv_0004']);
+  const none = await app.handle('GET', '/billing/invoices.csv?tenant=acme&currency=GBP');
+  assert.equal(none.status, 200);
+  assert.equal(none.body, HEADER + '\n');
+});
+
+test('server: invoices.csv errors', async () => {
+  const app = createApp();
+  const status = async (qs) => (await app.handle('GET', '/billing/invoices.csv' + qs)).status;
+  assert.equal(await status(''), 400);
+  assert.equal(await status('?tenant='), 400);
+  assert.equal(await status('?tenant=nobody'), 404);
+  assert.equal(await status('?tenant=acme&from=2026-13-01'), 400);
+  assert.equal(await status('?tenant=acme&to=yesterday'), 400);
+  assert.equal(await status('?tenant=acme&currency=JPY'), 400);
+  assert.equal(await status('?tenant=acme&currency=usd'), 400);
+  assert.equal(await status('?tenant=acme&currency='), 400);
+});