diff --git a/apps/web/public/static/css/style.css b/apps/web/public/static/css/style.css
index d3485eb..8b00818 100644
--- a/apps/web/public/static/css/style.css
+++ b/apps/web/public/static/css/style.css
@@ -776,6 +776,47 @@ a:hover {
   margin-top: 2.2rem;
 }
 
+/* ------------------------------------------------------------------- ads */
+
+/* One sponsored line, sold through CrawlProof. Sized here rather than by an ad
+   script, so it occupies the same space before and after it loads and moves
+   nothing underneath it.
+
+   An unsold slot answers with a blank document, and nothing on this side can
+   see that across an origin — so the unit is styled as a strip of whitespace
+   with no border, no ground and no label of its own. Empty, it reads as the gap
+   between two sections, which is where it was put. */
+.ad-unit {
+  display: block;
+  margin: 2.6rem 0;
+}
+.ad-unit iframe {
+  display: block;
+  border: 0;
+  max-width: 100%;
+}
+/* The fluid format: the creative lays itself out at whatever width it is given
+   and drops its second line on a narrow one. */
+.ad-unit[data-ad-format="text_link"] iframe {
+  width: 100%;
+  height: 40px;
+}
+/* The banner creatives are laid out at a fixed pixel width inside the frame, so
+   a narrowed frame would crop one rather than reflow it. They keep their own
+   width and centre instead. */
+.ad-unit:not([data-ad-format="text_link"]) iframe {
+  margin-inline: auto;
+}
+.ad-label {
+  display: block;
+  margin-bottom: 0.3rem;
+  font-family: var(--mono);
+  font-size: 0.6rem;
+  letter-spacing: 0.17em;
+  text-transform: uppercase;
+  color: var(--text-faint);
+}
+
 /* --------------------------------------------------------------- test page */
 
 /* Test pages run as a rack: the instrument list stays put, the bench changes. */
diff --git a/apps/web/src/app.js b/apps/web/src/app.js
index f0ebfba..503f006 100644
--- a/apps/web/src/app.js
+++ b/apps/web/src/app.js
@@ -35,7 +35,8 @@ app.use('*', async (c, next) => {
 
 /**
  * The analytics script, when one is configured, is the only third-party origin
- * the policy below has to make room for.
+ * the policy below has to make room for in `script-src`. The ad frame is the
+ * other third party, and it needs no script source at all.
  */
 const analyticsOrigin = (() => {
   if (!config.analytics.src) return null;
@@ -71,6 +72,20 @@ const CSP = [
   `connect-src 'self'${analyticsOrigin ? ` ${analyticsOrigin}` : ''}`,
   "worker-src 'self'",
   "manifest-src 'self'",
+  /**
+   * Advertising costs exactly one directive, and deliberately so. The ad is a
+   * plain cross-origin document in an iframe, which carries its own policy, so
+   * nothing else here has to move. The vendor's own snippet would have needed
+   * `script-src` for its tag, `connect-src` for the fetch behind it, and then —
+   * because it injects the creative as `srcdoc`, and a srcdoc document inherits
+   * the embedder's policy — `'unsafe-inline'` in `style-src` plus a wide-open
+   * `img-src` for every page on the site. See AdUnit.jsx.
+   *
+   * Omitted rather than set to 'none' when there is no slot: `default-src`
+   * already keeps frames to this origin, and a bare 'none' would be a rule
+   * about something nothing on the site does.
+   */
+  ...(config.ads.slot ? [`frame-src ${config.ads.origin}`] : []),
 ].join('; ');
 
 /**
diff --git a/apps/web/src/components/AdUnit.jsx b/apps/web/src/components/AdUnit.jsx
new file mode 100644
index 0000000..2573614
--- /dev/null
+++ b/apps/web/src/components/AdUnit.jsx
@@ -0,0 +1,62 @@
+import { config } from '@d3vices/config';
+
+/**
+ * One CrawlProof ad unit, embedded as a script-free cross-origin iframe.
+ *
+ * The generated install snippet loads `crawlproof.com/ad.js`, which fills each
+ * placeholder with a `srcdoc` iframe. A srcdoc document inherits the embedding
+ * page's Content-Security-Policy, so the creative's own `<style>` block and its
+ * remote images would render only if this site added `'unsafe-inline'` to
+ * `style-src` and opened `img-src` to the web — on every page, for an ad. The
+ * policy in app.js is a large part of what this site is; it is not worth that.
+ *
+ * `/api/ads/frame` returns the same creative as a real cross-origin document,
+ * which carries its own policy. This end needs one `frame-src` entry, runs no
+ * third-party JavaScript, sets no cookie, and — unlike the script tag, which
+ * mints a `crawlproof.visitor` id — writes nothing into this origin's local
+ * storage. The privacy page can go on saying what it says.
+ *
+ * The cost of having no script is that nothing here can measure the viewport,
+ * and every banner creative is laid out at a fixed pixel width inside the frame
+ * (a narrowed iframe clips it rather than reflowing it). `text_link` is the one
+ * format built to fill its container, so it is the default and the only one
+ * that belongs in a column whose width is not known in advance.
+ */
+const SIZES = {
+  banner_300x250: [300, 250],
+  banner_728x90: [728, 90],
+  banner_320x50: [320, 50],
+  // Fluid: the creative fills whatever width the frame is given.
+  text_link: [null, 40],
+};
+
+export function AdUnit({ format = 'text_link' }) {
+  const { origin, slot } = config.ads;
+  // No slot configured — the desktop export, and any checkout that has not set
+  // one — renders nothing at all rather than an empty box.
+  if (!slot) return null;
+  const size = SIZES[format];
+  if (!size) return null;
+  const [width, height] = size;
+  const src = `${origin}/api/ads/frame?slot=${encodeURIComponent(slot)}&format=${encodeURIComponent(format)}`;
+
+  return (
+    <aside class="ad-unit" data-ad-format={format}>
+      {/* The text link carries its own "Sponsored" mark inside the frame. The
+          banner creatives do not, so those get a label out here. */}
+      {format === 'text_link' ? null : <span class="ad-label">Advertisement</span>}
+      {/* No allow-scripts and no allow-same-origin: the creative is static
+          HTML. The popup permissions are what let its click-through open a new
+          tab, which is the only thing it is allowed to do. */}
+      <iframe
+        src={src}
+        title="Advertisement"
+        width={width ?? undefined}
+        height={height}
+        loading="lazy"
+        scrolling="no"
+        sandbox="allow-popups allow-popups-to-escape-sandbox allow-top-navigation-by-user-activation"
+      />
+    </aside>
+  );
+}
diff --git a/apps/web/src/export.js b/apps/web/src/export.js
index d2cec0b..35468f8 100644
--- a/apps/web/src/export.js
+++ b/apps/web/src/export.js
@@ -6,8 +6,21 @@
  */
 import { cp, mkdir, rm, writeFile } from 'node:fs/promises';
 import { dirname, join } from 'node:path';
-import { TESTS } from '@d3vices/tests/registry';
-import app from './app.js';
+
+// The desktop app ships this output and makes no network request unless you run
+// the network test — a promise the privacy page makes on its behalf. An ad frame
+// is a network request, so advertising is switched off here rather than left to
+// whoever runs the build.
+//
+// Everything that could reach the config is imported after the assignment, and
+// dynamically: a static import is evaluated before the first statement in this
+// file, so the config would have read the variable as it was and the desktop
+// build would have quietly shipped ads.
+process.env.ADS_SLOT = '';
+const [{ TESTS }, { default: app }] = await Promise.all([
+  import('@d3vices/tests/registry'),
+  import('./app.js'),
+]);
 
 const outDir = process.argv[2] || join(import.meta.dir, '../../../dist/site');
 
diff --git a/apps/web/src/pages/Home.jsx b/apps/web/src/pages/Home.jsx
index 88dd249..26361df 100644
--- a/apps/web/src/pages/Home.jsx
+++ b/apps/web/src/pages/Home.jsx
@@ -1,5 +1,6 @@
 import { buildDate, config } from '@d3vices/config';
 import { GROUPS, TESTS, testsInGroup } from '@d3vices/tests/registry';
+import { AdUnit } from '../components/AdUnit.jsx';
 import { Layout } from '../components/Layout.jsx';
 
 /** Questions about the site rather than about one instrument. */
@@ -179,6 +180,12 @@ export function Home() {
         ))}
       </section>
 
+      {/* After the rack, not before it. Whoever came here to test something has
+          already been handed the whole list; this sits in the seam between the
+          instruments and the copy about them, where a section break was going
+          to be anyway. */}
+      <AdUnit />
+
       <section class="section section-split">
         <div>
           <h2>Why another device tester?</h2>
diff --git a/apps/web/src/pages/Privacy.jsx b/apps/web/src/pages/Privacy.jsx
index d46436e..51ca9c0 100644
--- a/apps/web/src/pages/Privacy.jsx
+++ b/apps/web/src/pages/Privacy.jsx
@@ -40,10 +40,30 @@ export function Privacy() {
 
         <h2>Third parties</h2>
         <p>
-          {config.analytics.src
-            ? 'A privacy-preserving, cookie-free analytics endpoint records page views in aggregate. It sets no cookie and does not fingerprint you.'
-            : 'None. No analytics, no advertising, no fonts or scripts loaded from anyone else’s server.'}
+          The two typefaces are loaded from Google Fonts, so fetching them tells Google that a browser at your
+          address asked for a font file. Nothing else about you goes with it.
         </p>
+        {config.analytics.src ? (
+          <p>
+            A privacy-preserving, cookie-free analytics endpoint records page views in aggregate. It sets no
+            cookie and does not fingerprint you.
+          </p>
+        ) : null}
+        {config.ads.slot ? (
+          <p>
+            There is one advertisement on some pages, sold through CrawlProof, and it is what pays for the
+            site. It is embedded the quiet way: a plain frame containing a page from their server, rather than
+            the usual advertising script. No third-party code runs on this page, no cookie is set, and nothing
+            is written to the local storage described above — an ad script would have put a permanent visitor
+            id there, and that is precisely why there is no ad script. Loading the frame tells CrawlProof what
+            any request tells a server: your IP address, your browser’s user agent, and which page the ad
+            appeared on. Which ad you get is chosen from that and nothing else. No reading any test takes is
+            available to it, or to anyone.
+          </p>
+        ) : null}
+        {!config.analytics.src && !config.ads.slot ? (
+          <p>Nothing further. No analytics and no advertising.</p>
+        ) : null}
 
         <h2>The desktop app</h2>
         <p>
diff --git a/apps/web/src/pages/TestPage.jsx b/apps/web/src/pages/TestPage.jsx
index b5e401a..9309171 100644
--- a/apps/web/src/pages/TestPage.jsx
+++ b/apps/web/src/pages/TestPage.jsx
@@ -1,6 +1,7 @@
 import { config } from '@d3vices/config';
 import { API_DOCS } from '@d3vices/tests/api-docs';
 import { TESTS, testsInGroup } from '@d3vices/tests/registry';
+import { AdUnit } from '../components/AdUnit.jsx';
 import { Layout } from '../components/Layout.jsx';
 import { RackNav, rackCode } from '../components/RackNav.jsx';
 
@@ -123,6 +124,13 @@ export function TestPage({ test }) {
             </section>
           ) : null}
 
+          {/* Below the instrument, its permissions and its FAQ: the reading is
+              done and the next thing on the page is a list of links away, so a
+              sponsored line here interrupts nothing. Never above the test — a
+              reader who came to find out whether their microphone works should
+              reach it without passing an ad. */}
+          <AdUnit />
+
           <section class="related">
             <h2>Nearby instruments</h2>
             <div class="card-grid">
diff --git a/packages/config/src/index.js b/packages/config/src/index.js
index 5de78e7..e1c7b9e 100644
--- a/packages/config/src/index.js
+++ b/packages/config/src/index.js
@@ -13,6 +13,19 @@ export const config = {
     src: env('ANALYTICS_SRC', ''),
     siteId: env('ANALYTICS_SITE_ID', ''),
   },
+  /**
+   * The CrawlProof slot this site sells its ad inventory through. It is a
+   * public identifier, not a secret — it ships in the markup — so it is written
+   * down here rather than left to a variable somebody has to remember to set,
+   * which would make merging this a silent no-op.
+   *
+   * Set `ADS_SLOT=` (empty) to turn advertising off. The static export does
+   * exactly that, because the desktop app must make no network request.
+   */
+  ads: {
+    origin: 'https://crawlproof.com',
+    slot: env('ADS_SLOT', 'f54a21ad-3db6-4a62-a9f4-93d505c221c9'),
+  },
 };
 
 export const siteName = 'd3vices';