diff --git a/assets/app.js b/assets/app.js
index 86f4d35..c4b7d3e 100644
--- a/assets/app.js
+++ b/assets/app.js
@@ -20,6 +20,15 @@ const modalInput = document.getElementById('modalInput');
 const modalTitle = document.getElementById('modalTitle');
 const hapticTick = () => { if ('vibrate' in navigator) navigator.vibrate(12); };
 
+// --- Service Worker ---
+if ('serviceWorker' in navigator) {
+    window.addEventListener('load', () => {
+        navigator.serviceWorker.register('/sw.js').catch(() => {
+            // fail silently; optional log
+        });
+    });
+}
+
 // --- Sweet-ish Alerts ---
 function removeSwal() {
     const existing = document.querySelector('.swal-overlay');
diff --git a/assets/site.webmanifest b/assets/site.webmanifest
index 4e00e86..d7c60cc 100644
--- a/assets/site.webmanifest
+++ b/assets/site.webmanifest
@@ -1,6 +1,8 @@
 {
   "name": "Toadstool Tally",
   "short_name": "Toadstool",
+  "start_url": ".",
+  "scope": ".",
   "icons": [
     {
       "src": "icons/web-app-manifest-192x192.png",
diff --git a/sw.js b/sw.js
new file mode 100644
index 0000000..b4308db
--- /dev/null
+++ b/sw.js
@@ -0,0 +1,50 @@
+const CACHE_NAME = 'toadstool-tally-v1';
+const ASSETS = [
+  '/',
+  '/index.html',
+  '/assets/style.css',
+  '/assets/app.js',
+  '/assets/textures/mushroom.svg',
+  '/assets/icons/favicon-96x96.png',
+  '/assets/icons/favicon.ico',
+  '/assets/icons/favicon.svg',
+  '/assets/icons/apple-touch-icon.png',
+  '/assets/icons/web-app-manifest-192x192.png',
+  '/assets/icons/web-app-manifest-512x512.png',
+  '/assets/site.webmanifest'
+];
+
+self.addEventListener('install', (event) => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then((cache) => cache.addAll(ASSETS))
+  );
+});
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    caches.keys().then((keys) =>
+      Promise.all(
+        keys
+          .filter((key) => key !== CACHE_NAME)
+          .map((key) => caches.delete(key))
+      )
+    )
+  );
+  self.clients.claim();
+});
+
+self.addEventListener('fetch', (event) => {
+  const { request } = event;
+  if (request.method !== 'GET') return;
+
+  event.respondWith(
+    caches.match(request).then((cached) => {
+      if (cached) return cached;
+      return fetch(request).then((response) => {
+        const copy = response.clone();
+        caches.open(CACHE_NAME).then((cache) => cache.put(request, copy));
+        return response;
+      }).catch(() => cached);
+    })
+  );
+});