diff --git a/CHANGES.md b/CHANGES.md
index 7539f2e..e9d0eff 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased
+- Billing: `GET /billing/invoices.csv?tenant=&from=&to=&currency=` exports a tenant's invoices as CSV; `currency` (USD, EUR or GBP) optionally filters the rows.
 
 ## 3.2.0
 - Returns: partial refunds.
diff --git a/docs/architecture.md b/docs/architecture.md
index fdb6191..e398bb6 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -4,4 +4,5 @@ Each module registers routes with the router in `src/lib/router.js`. Handlers re
 `{ params, query, body, tenantId }` and return `{ status, body, headers }` through the
 helpers in `src/lib/http.js`. Money is always integer cents; format it only with
 `formatCents()` from `src/lib/money.js`. Errors thrown from services are mapped to
-HTTP statuses by `src/lib/errors.js`.
+HTTP statuses by `src/lib/errors.js`. Non-JSON responses use `text()` from the same
+helpers; `src/lib/csv.js` holds the RFC 4180 CSV quoting.
diff --git a/src/lib/csv.js b/src/lib/csv.js
new file mode 100644
index 0000000..c117818
--- /dev/null
+++ b/src/lib/csv.js
@@ -0,0 +1,10 @@
+// RFC 4180 field quoting.
+export function csvField(v) {
+  const s = v === null || v === undefined ? '' : String(v);
+  return /[",\r\n]/.test(s) ? '"' + s.replace(/"/g, '""') + '"' : s;
+}
+
+// Rows are arrays of fields. Every line, including the last, ends with a newline.
+export function toCsv(rows) {
+  return rows.map((r) => r.map(csvField).join(',') + '\n').join('');
+}
diff --git a/src/lib/validate.js b/src/lib/validate.js
index 0fe84c3..b2161ce 100644
--- a/src/lib/validate.js
+++ b/src/lib/validate.js
@@ -14,6 +14,16 @@ export function optionalInt(obj, key, { min = 0, max = Number.MAX_SAFE_INTEGER }
   return v;
 }
 
+// Strict YYYY-MM-DD calendar date. Absent or empty means undefined.
+export function optionalDate(obj, key) {
+  const v = obj?.[key];
+  if (v === undefined || v === null || v === '') return undefined;
+  const t = typeof v === 'string' && /^\d{4}-\d{2}-\d{2}$/.test(v) ? Date.parse(v + 'T00:00:00Z') : NaN;
+  // Date.parse yields NaN for month 13 etc.; the round trip catches rollovers like 02-31.
+  if (Number.isNaN(t) || new Date(t).toISOString().slice(0, 10) !== v) throw new ValidationError(`${key} must be a date in YYYY-MM-DD format`);
+  return v;
+}
+
 export function oneOf(obj, key, allowed) {
   const v = obj?.[key];
   if (!allowed.includes(v)) throw new ValidationError(`${key} must be one of ${allowed.join(', ')}`);
diff --git a/src/modules/billing/routes.js b/src/modules/billing/routes.js
index 0ac3d5c..4ce150f 100644
--- a/src/modules/billing/routes.js
+++ b/src/modules/billing/routes.js
@@ -1,8 +1,9 @@
-import { ok, wrap } from '../../lib/http.js';
-import { getInvoice, listInvoices } from './service.js';
+import { ok, text, wrap } from '../../lib/http.js';
+import { exportInvoicesCsv, getInvoice, listInvoices } from './service.js';
 import { formatInvoice } from './formatters.js';
 
 export const routes = [
   ['GET', '/billing/invoices', wrap(async (req) => ok(listInvoices(req.query).map(formatInvoice)))],
+  ['GET', '/billing/invoices.csv', wrap(async (req) => text(exportInvoicesCsv(req.query), '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..5179935 100644
--- a/src/modules/billing/service.js
+++ b/src/modules/billing/service.js
@@ -1,4 +1,8 @@
 import { NotFoundError } from '../../lib/errors.js';
+import { toCsv } from '../../lib/csv.js';
+import { formatCents } from '../../lib/money.js';
+import { inRange } from '../../lib/time.js';
+import { oneOf, optionalDate, requireString } from '../../lib/validate.js';
 import { discountFor } from './discounts.js';
 import { taxFor } from './tax.js';
 import { TAX_REGION } from './constants.js';
@@ -21,3 +25,30 @@ export function getInvoice(id) {
 export function listInvoices({ tenantId } = {}) {
   return allInvoices().filter((i) => !tenantId || i.tenantId === tenantId);
 }
+
+const CURRENCIES = ['USD', 'EUR', 'GBP'];
+const CSV_HEADER = ['id', 'date', 'customer', 'subtotal', 'discount', 'tax', 'total', 'currency'];
+
+// CSV of one tenant's invoices. query: { tenant, from?, to?, currency? }.
+export function exportInvoicesCsv(query) {
+  const tenantId = requireString(query, 'tenant');
+  const from = optionalDate(query, 'from');
+  const to = optionalDate(query, 'to');
+  const currency = query?.currency ? oneOf(query, 'currency', CURRENCIES) : undefined;
+  const all = listInvoices({ tenantId });
+  if (all.length === 0) throw new NotFoundError('tenant ' + tenantId);
+  const rows = all
+    .filter((i) => inRange(i.date, from, to) && (!currency || i.currency === currency))
+    .sort((a, b) => (a.date < b.date ? -1 : a.date > b.date ? 1 : a.id < b.id ? -1 : a.id > b.id ? 1 : 0))
+    .map((i) => [
+      i.id,
+      i.date,
+      i.customer,
+      formatCents(i.subtotal),
+      formatCents(i.discount),
+      formatCents(i.tax),
+      formatCents(i.total),
+      i.currency,
+    ]);
+  return toCsv([CSV_HEADER, ...rows]);
+}
diff --git a/tests/billing-csv.test.js b/tests/billing-csv.test.js
new file mode 100644
index 0000000..9898fea
--- /dev/null
+++ b/tests/billing-csv.test.js
@@ -0,0 +1,98 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { createApp } from '../src/server.js';
+import { csvField, toCsv } from '../src/lib/csv.js';
+import { optionalDate } from '../src/lib/validate.js';
+
+const HEADER = 'id,date,customer,subtotal,discount,tax,total,currency\n';
+const get = (qs) => createApp().handle('GET', '/billing/invoices.csv' + qs);
+const ids = (body) => body.split('\n').slice(1, -1).map((l) => l.split(',')[0]);
+
+test('csv: field quoting', () => {
+  assert.equal(csvField('plain'), 'plain');
+  assert.equal(csvField('a,b'), '"a,b"');
+  assert.equal(csvField('say "hi"'), '"say ""hi"""');
+  assert.equal(csvField('two\nlines'), '"two\nlines"');
+  assert.equal(csvField('cr\rhere'), '"cr\rhere"');
+  assert.equal(csvField(null), '');
+  assert.equal(csvField(undefined), '');
+  assert.equal(csvField(12), '12');
+  assert.equal(toCsv([['a', 'b,c'], ['d', 'e']]), 'a,"b,c"\nd,e\n');
+});
+
+test('validate: optionalDate', () => {
+  assert.equal(optionalDate({}, 'from'), undefined);
+  assert.equal(optionalDate({ from: '' }, 'from'), undefined);
+  assert.equal(optionalDate({ from: '2026-09-15' }, 'from'), '2026-09-15');
+  for (const bad of ['2026-02-31', '2026-13-01', '15-09-2026', '2026-9-5', 'garbage', ' 2026-09-15']) {
+    assert.throws(() => optionalDate({ from: bad }, 'from'), { status: 400 }, bad);
+  }
+});
+
+test('billing csv: columns, money format, sort order, content type', async () => {
+  const r = await get('?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\n' +
+      'inv_0001,2026-09-02,Blue Fern Ltd,120.00,12.00,7.56,115.56,USD\n' +
+      'inv_0002,2026-09-15,"Smith, Jones & Co",49.99,0.00,3.50,53.49,USD\n' +
+      'inv_0003,2026-09-15,"The ""Quick"" Shop",200.00,5.00,39.00,234.00,EUR\n' +
+      'inv_0004,2026-10-01,Ortega Freight,1.05,0.00,0.07,1.12,USD\n',
+  );
+});
+
+test('billing csv: tenants are isolated', async () => {
+  const r = await get('?tenant=globex');
+  assert.equal(r.body, HEADER + 'inv_0005,2026-09-20,Northwind,80.00,8.00,14.40,86.40,GBP\n');
+});
+
+test('billing csv: from and to are inclusive and optional', async () => {
+  assert.deepEqual(ids((await get('?tenant=acme&from=2026-09-15&to=2026-09-15')).body), ['inv_0002', 'inv_0003']);
+  assert.deepEqual(ids((await get('?tenant=acme&from=2026-09-15')).body), ['inv_0002', 'inv_0003', 'inv_0004']);
+  assert.deepEqual(ids((await get('?tenant=acme&to=2026-09-02')).body), ['inv_0006', 'inv_0001']);
+  assert.deepEqual(ids((await get('?tenant=acme&from=&to=')).body).length, 5);
+});
+
+test('billing csv: currency filter', async () => {
+  assert.deepEqual(ids((await get('?tenant=acme&currency=EUR')).body), ['inv_0003']);
+  assert.deepEqual(ids((await get('?tenant=acme&currency=USD&from=2026-09-15')).body), ['inv_0002', 'inv_0004']);
+  assert.equal((await get('?tenant=acme&currency=GBP')).body, HEADER);
+  assert.equal(ids((await get('?tenant=acme&currency=')).body).length, 5);
+});
+
+test('billing csv: invalid currency is a 400, before the tenant lookup', async () => {
+  for (const bad of ['JPY', 'usd', 'USD,EUR', '%20USD']) {
+    assert.equal((await get('?tenant=acme&currency=' + bad)).status, 400, bad);
+  }
+  assert.equal((await get('?tenant=nobody&currency=JPY')).status, 400);
+  assert.equal((await get('?tenant=nobody&currency=USD')).status, 404);
+});
+
+test('billing csv: no match in range returns just the header', async () => {
+  assert.equal((await get('?tenant=acme&from=2027-01-01')).body, HEADER);
+  const r = await get('?tenant=acme&from=2026-10-01&to=2026-09-01');
+  assert.equal(r.status, 200);
+  assert.equal(r.body, HEADER);
+});
+
+test('billing csv: errors', async () => {
+  assert.equal((await get('')).status, 400);
+  assert.equal((await get('?tenant=')).status, 400);
+  assert.equal((await get('?tenant=%20%20')).status, 400);
+  assert.equal((await get('?from=2026-09-01')).status, 400);
+  assert.equal((await get('?tenant=acme&from=2026-02-31')).status, 400);
+  assert.equal((await get('?tenant=acme&to=garbage')).status, 400);
+  assert.equal((await get('?tenant=acme&from=2026-13-01')).status, 400);
+  assert.equal((await get('?tenant=nobody')).status, 404);
+  // validation happens before the tenant lookup
+  assert.equal((await get('?tenant=nobody&from=garbage')).status, 400);
+});
+
+test('billing csv: existing invoice routes still work', async () => {
+  const app = createApp();
+  assert.equal((await app.handle('GET', '/billing/invoices/inv_0004')).status, 200);
+  assert.equal((await app.handle('GET', '/billing/invoices')).status, 200);
+});