diff --git a/src/app/globals.css b/src/app/globals.css
index 87cf654..8fe8fd2 100644
--- a/src/app/globals.css
+++ b/src/app/globals.css
@@ -128,3 +128,8 @@ dl.facts dd { margin: 0; overflow-wrap: anywhere; }
 .site-footer nav { display: flex; gap: 1rem; flex-wrap: wrap; margin-bottom: 0.75rem; }
 .site-footer a { color: var(--muted); }
 .site-footer + div > .pfs-footer { border-top: 0; }
+
+/* CrawlProof ad unit: no box until ad.js starts filling it, so an unsold or blocked slot leaves no hole. */
+.ad-unit { display: flex; flex-direction: column; align-items: center; height: 0; overflow: hidden; }
+.ad-unit[data-cp-filled] { height: auto; min-height: 106px; margin: 1.5rem 0; overflow: visible; }
+.ad-unit[data-cp-filled][data-format="banner_300x250"] { min-height: 266px; }
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index 4b57f8c..55b0411 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -1,6 +1,7 @@
 import type { Metadata, Viewport } from "next";
 import { Footer } from "@profullstack/footer/react";
 import Link from "next/link";
+import { AdUnit } from "@/components/ad-unit";
 import { RegisterServiceWorker } from "@/components/register-sw";
 import { DISCLOSURE } from "@/lib/site";
 import "./globals.css";
@@ -46,6 +47,7 @@ export default function RootLayout({ children }: { children: React.ReactNode })
       <head>
         {/* CrawlProof stats: a plain async tag so it is in the served HTML (next/script afterInteractive is not). */}
         <script data-site="6be9abdb-fd93-4756-bbf4-3ebefc3117c3" src="https://crawlproof.com/stats.js" async></script>
+        <script src="https://crawlproof.com/ad.js" async></script>
       </head>
       <body>
         <a className="skip" href="#main">Skip to content</a>
@@ -63,6 +65,9 @@ export default function RootLayout({ children }: { children: React.ReactNode })
           </div>
         </header>
         <main id="main" className="wrap">{children}</main>
+        <div className="wrap">
+          <AdUnit />
+        </div>
         <div className="site-footer">
           <div className="wrap">
             <p>{DISCLOSURE}</p>
diff --git a/src/components/ad-unit.tsx b/src/components/ad-unit.tsx
new file mode 100644
index 0000000..e608896
--- /dev/null
+++ b/src/components/ad-unit.tsx
@@ -0,0 +1,44 @@
+"use client";
+
+import { useEffect, useRef } from "react";
+
+/** CrawlProof ad slot for datamart.help. Manage it at https://crawlproof.com/ads/slots */
+const AD_SLOT = "04392e64-733a-48dd-b17b-c4dc9eb3a399";
+
+/** 728px banner plus the .wrap gutters; anything narrower gets the 300x250 rectangle. */
+const LEADERBOARD_QUERY = "(min-width: 760px)";
+
+declare global {
+  interface Window {
+    crawlproofAds?: { scan: () => void };
+  }
+}
+
+/**
+ * One CrawlProof unit above the footer. The aside is server-rendered without
+ * data-slot, so ad.js's own DOMContentLoaded pass skips it; the format is picked
+ * from the real viewport on mount, then we ask ad.js to scan (retrying briefly
+ * until it has loaded, and giving up so an ad blocker leaves no timer running).
+ * It stays zero-height until ad.js marks it data-cp-filled.
+ */
+export function AdUnit() {
+  const ref = useRef<HTMLElement>(null);
+
+  useEffect(() => {
+    const el = ref.current;
+    if (!el) return;
+    el.dataset.format = window.matchMedia(LEADERBOARD_QUERY).matches ? "banner_728x90" : "banner_300x250";
+    el.dataset.slot = AD_SLOT;
+
+    let tries = 0;
+    let timer: ReturnType<typeof setTimeout> | undefined;
+    const tick = () => {
+      if (window.crawlproofAds) return window.crawlproofAds.scan();
+      if (++tries <= 25) timer = setTimeout(tick, 200);
+    };
+    tick();
+    return () => clearTimeout(timer);
+  }, []);
+
+  return <aside ref={ref} className="ad-unit" data-cp-ad="" aria-label="Advertisement" />;
+}