diff --git a/css/style.css b/css/style.css
index 0809380..b0cc4f4 100644
--- a/css/style.css
+++ b/css/style.css
@@ -115,9 +115,32 @@ html, body {
 #layers {
   display: flex;
   flex-direction: column;
-  gap: 2px;
+  gap: 1px;
   overflow-y: auto;
 }
+
+/* ---- collapsible category sections ---- */
+.cat:has(.cat-body:empty) { display: none; } /* hide unused categories */
+.cat-header {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  padding: 4px 4px 3px;
+  margin-top: 4px;
+  cursor: pointer;
+  user-select: none;
+  color: var(--dim);
+  font-size: 9.5px;
+  letter-spacing: 0.6px;
+  text-transform: uppercase;
+  border-top: 1px solid var(--border);
+}
+.cat-header:hover { color: var(--text); }
+.cat-chevron { font-size: 8px; width: 8px; transition: transform 0.15s; }
+.cat.collapsed .cat-chevron { transform: rotate(-90deg); }
+.cat.collapsed .cat-body { display: none; }
+.cat-body { display: flex; flex-direction: column; gap: 1px; }
+
 .layer-row {
   padding: 5px 4px;
   border-radius: 6px;
diff --git a/js/ui.js b/js/ui.js
index 731fabc..0bf14f8 100644
--- a/js/ui.js
+++ b/js/ui.js
@@ -1,9 +1,60 @@
-// HUD controller: layer registry, per-feed status lines, LIVE/SCRUBBED chip.
+// HUD controller: categorized collapsible layer list, per-feed status lines,
+// LIVE/SCRUBBED chip.
 
 const rows = new Map();
 
+// Category scheme is owned here so HUD order is stable regardless of layer
+// registration order. Categories collapsed-by-default are marked below.
+const CATEGORY_ORDER = ['Space', 'Air', 'Sea', 'Earth & hazards', 'Context', 'Regional — Australia'];
+const COLLAPSED_BY_DEFAULT = new Set(['Regional — Australia']);
+const CATEGORY_BY_ID = {
+  satellites: 'Space', 'sat-paths': 'Space', launches: 'Space',
+  aircraft: 'Air', military: 'Air', 'aircraft-mil': 'Air',
+  ships: 'Sea',
+  quakes: 'Earth & hazards', fires: 'Earth & hazards', gdacs: 'Earth & hazards', nws: 'Earth & hazards',
+  infra: 'Context', events: 'Context',
+  rfs: 'Regional — Australia',
+};
+
+const sections = new Map(); // category → { body }
+
+// Lazily create a category section, inserted in CATEGORY_ORDER position.
+function sectionFor(category) {
+  if (sections.has(category)) return sections.get(category);
+  const wrap = document.createElement('div');
+  wrap.className = 'cat';
+  if (COLLAPSED_BY_DEFAULT.has(category)) wrap.classList.add('collapsed');
+  const header = document.createElement('div');
+  header.className = 'cat-header';
+  header.innerHTML = `<span class="cat-chevron">▾</span><span class="cat-name"></span>`;
+  header.querySelector('.cat-name').textContent = category;
+  header.addEventListener('click', () => wrap.classList.toggle('collapsed'));
+  const body = document.createElement('div');
+  body.className = 'cat-body';
+  wrap.appendChild(header);
+  wrap.appendChild(body);
+
+  // Insert before the existing section that is this one's immediate successor in
+  // CATEGORY_ORDER (smallest index greater than ours), so DOM order stays stable
+  // regardless of registration order. Unknown categories sort last.
+  const container = document.getElementById('layers');
+  const ord = (c) => { const i = CATEGORY_ORDER.indexOf(c); return i < 0 ? Infinity : i; };
+  const myOrder = ord(category);
+  let before = null, bestOrder = Infinity;
+  for (const [cat, s] of sections) {
+    const idx = ord(cat);
+    if (idx > myOrder && idx < bestOrder) { bestOrder = idx; before = s.wrap; }
+  }
+  container.insertBefore(wrap, before);
+  const rec = { wrap, body };
+  sections.set(category, rec);
+  return rec;
+}
+
 // Register a layer row. Returns a handle so callers can flip the checkbox.
-export function addLayer(id, name, defaultOn, onToggle) {
+// `category` is optional; a known id maps to a fixed category regardless.
+export function addLayer(id, name, defaultOn, onToggle, category) {
+  const cat = CATEGORY_BY_ID[id] || category || 'Other';
   const row = document.createElement('div');
   row.className = 'layer-row';
   row.innerHTML = `
@@ -16,7 +67,7 @@ export function addLayer(id, name, defaultOn, onToggle) {
   row.querySelector('.lname').textContent = name;
   const cb = row.querySelector('input');
   cb.addEventListener('change', () => onToggle(cb.checked));
-  document.getElementById('layers').appendChild(row);
+  sectionFor(cat).body.appendChild(row);
   rows.set(id, row);
   return { setChecked: (v) => { cb.checked = v; } };
 }