diff --git a/CHANGES.md b/CHANGES.md
index 7539f2e..1ea1c24 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,7 @@
 # Changes
 
 ## Unreleased
+- Billing: CSV export of a tenant's invoices at 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..82c3889
--- /dev/null
+++ b/src/lib/csv.js
@@ -0,0 +1,14 @@
+// RFC 4180 field quoting: quote when needed, double inner quotes.
+export function csvField(v) {
+  const s = v == null ? '' : String(v);
+  return /[",\r\n]/.test(s) ? `"${s.replace(/"/g, '""')}"` : s;
+}
+
+export function csvRow(fields) {
+  return fields.map(csvField).join(',');
+}
+
+// Rows are arrays of fields. Lines end with \n, including the last one.
+export function toCsv(rows) {
+  return rows.map((r) => csvRow(r) + '\n').join('');
+}
diff --git a/src/lib/validate.js b/src/lib/validate.js
index 0fe84c3..2508577 100644
--- a/src/lib/validate.js
+++ b/src/lib/validate.js
@@ -14,6 +14,18 @@ export function optionalInt(obj, key, { min = 0, max = Number.MAX_SAFE_INTEGER }
   return v;
 }
 
+// Optional YYYY-MM-DD date. Absent or empty means undefined.
+export function optionalIsoDate(obj, key) {
+  const v = obj?.[key];
+  if (v === undefined || v === null || v === '') return undefined;
+  const d = typeof v === 'string' && /^\d{4}-\d{2}-\d{2}$/.test(v) ? new Date(v) : null;
+  // Round-trip: Date rolls over 2026-02-30 to March instead of rejecting it.
+  if (!d || isNaN(d) || d.toISOString().slice(0, 10) !== v) {
+    throw new ValidationError(`${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..a4ff88e 100644
--- a/src/modules/billing/routes.js
+++ b/src/modules/billing/routes.js
@@ -1,8 +1,10 @@
-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/:id', wrap(async (req) => ok(formatInvoice(getInvoice(req.params.id))))],
+  // After :id on purpose: the router leaves '.' unescaped, so registered first this would also match /billing/invoices/csv.
+  ['GET', '/billing/invoices.csv', wrap(async (req) => text(exportInvoicesCsv(req.query), 'text/csv'))],
 ];
diff --git a/src/modules/billing/service.js b/src/modules/billing/service.js
index d84492f..61bab1b 100644
--- a/src/modules/billing/service.js
+++ b/src/modules/billing/service.js
@@ -1,8 +1,13 @@
 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 { optionalIsoDate, requireString } from '../../lib/validate.js';
 import { discountFor } from './discounts.js';
 import { taxFor } from './tax.js';
 import { TAX_REGION } from './constants.js';
 import { allInvoices, findInvoice } from './repo.js';
+import { validateCurrency } from './validators.js';
 
 // Prices order lines. Returns integer cents.
 export function priceLines(lines, { discountCode, region = TAX_REGION } = {}) {
@@ -21,3 +26,29 @@ export function getInvoice(id) {
 export function listInvoices({ tenantId } = {}) {
   return allInvoices().filter((i) => !tenantId || i.tenantId === tenantId);
 }
+
+const CSV_HEADER = ['id', 'date', 'customer', 'subtotal', 'discount', 'tax', 'total', 'currency'];
+
+// CSV export of one tenant's invoices. query: { tenant, from?, to?, currency? }.
+export function exportInvoicesCsv(query) {
+  const tenantId = requireString(query, 'tenant');
+  const from = optionalIsoDate(query, 'from');
+  const to = optionalIsoDate(query, 'to');
+  const currency = query.currency ? validateCurrency(query) : undefined;
+  const all = listInvoices({ tenantId });
+  if (all.length === 0) throw new NotFoundError('invoices for 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/lib.test.js b/tests/lib.test.js
index a03e86e..1caa347 100644
--- a/tests/lib.test.js
+++ b/tests/lib.test.js
@@ -2,6 +2,26 @@ 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';
+import { optionalIsoDate } from '../src/lib/validate.js';
+
+test('lib: csv', () => {
+  assert.equal(csvField('plain'), 'plain');
+  assert.equal(csvField(null), '');
+  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,c'], ['d', 'e']]), 'a,"b,c"\nd,e\n');
+});
+
+test('lib: optionalIsoDate', () => {
+  assert.equal(optionalIsoDate({}, 'from'), undefined);
+  assert.equal(optionalIsoDate({ from: '' }, 'from'), undefined);
+  assert.equal(optionalIsoDate({ from: '2026-09-15' }, 'from'), '2026-09-15');
+  for (const bad of ['2026-02-30', '2026-13-01', '2026-9-1', 'junk', '2026-09-15T00:00:00Z']) {
+    assert.throws(() => optionalIsoDate({ from: bad }, 'from'), /from must be a date/);
+  }
+});
 
 test('lib: money', () => {
   assert.equal(formatCents(105), '1.05');
diff --git a/tests/server.test.js b/tests/server.test.js
index e836eaa..7b69ece 100644
--- a/tests/server.test.js
+++ b/tests/server.test.js
@@ -17,3 +17,48 @@ 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\n';
+const csv = (qs) => createApp().handle('GET', '/billing/invoices.csv' + qs);
+const ids = (r) => r.body.split('\n').slice(1, -1).map((l) => l.split(',')[0]);
+
+test('csv export: validation and 404s', async () => {
+  assert.equal((await csv('')).status, 400);
+  assert.equal((await csv('?tenant=%20')).status, 400);
+  assert.equal((await csv('?tenant=acme&from=2026-02-30')).status, 400);
+  assert.equal((await csv('?tenant=acme&to=2026-13-01')).status, 400);
+  assert.equal((await csv('?tenant=nobody&from=junk')).status, 400);
+  assert.equal((await csv('?tenant=nobody')).status, 404);
+  assert.equal((await createApp().handle('GET', '/billing/invoices/csv')).status, 404);
+});
+
+test('csv export: format, order and tenant isolation', async () => {
+  const g = await csv('?tenant=globex');
+  assert.equal(g.status, 200);
+  assert.equal(g.headers['content-type'], 'text/csv');
+  assert.equal(g.body, HEADER + 'inv_0005,2026-09-20,Northwind,80.00,8.00,14.40,86.40,GBP\n');
+  const a = await csv('?tenant=acme');
+  assert.deepEqual(ids(a), ['inv_0006', 'inv_0001', 'inv_0002', 'inv_0003', 'inv_0004']);
+  assert.ok(a.body.endsWith('\n'));
+  assert.ok(a.body.includes('inv_0002,2026-09-15,"Smith, Jones & Co",49.99,0.00,3.50,53.49,USD\n'));
+  assert.ok(a.body.includes('inv_0003,2026-09-15,"The ""Quick"" Shop",200.00,5.00,39.00,234.00,EUR\n'));
+  assert.ok(a.body.includes('inv_0004,2026-10-01,Ortega Freight,1.05,0.00,0.07,1.12,USD\n'));
+});
+
+test('csv export: currency filter', async () => {
+  assert.deepEqual(ids(await csv('?tenant=acme&currency=USD')), ['inv_0006', 'inv_0001', 'inv_0002', 'inv_0004']);
+  assert.deepEqual(ids(await csv('?tenant=acme&currency=EUR')), ['inv_0003']);
+  assert.deepEqual(ids(await csv('?tenant=acme&currency=USD&from=2026-09-10')), ['inv_0002', 'inv_0004']);
+  assert.equal((await csv('?tenant=acme&currency=GBP')).body, HEADER);
+  assert.equal((await csv('?tenant=acme&currency=')).status, 200);
+  assert.equal((await csv('?tenant=acme&currency=JPY')).status, 400);
+  assert.equal((await csv('?tenant=acme&currency=usd')).status, 400);
+  assert.equal((await csv('?tenant=nobody&currency=JPY')).status, 400);
+});
+
+test('csv export: inclusive range and empty result', async () => {
+  assert.deepEqual(ids(await csv('?tenant=acme&from=2026-09-15&to=2026-09-15')), ['inv_0002', 'inv_0003']);
+  assert.deepEqual(ids(await csv('?tenant=acme&from=2026-09-01&to=2026-10-01')), ['inv_0001', 'inv_0002', 'inv_0003', 'inv_0004']);
+  assert.equal((await csv('?tenant=acme&from=2030-01-01')).body, HEADER);
+  assert.equal((await csv('?tenant=acme&from=2026-10-01&to=2026-09-01')).body, HEADER);
+});