diff --git a/e2e/app.spec.ts b/e2e/app.spec.ts
index 200535d3..c79cbd75 100644
--- a/e2e/app.spec.ts
+++ b/e2e/app.spec.ts
@@ -10,9 +10,13 @@ test('app loads with four-panel layout', async ({ page }) => {
   await expect(page.locator('.inspector')).toBeVisible()
 })
 
-test('sidebar shows navigation items', async ({ page }) => {
+test('sidebar shows section groups', async ({ page }) => {
   await page.goto('/')
+  await page.waitForTimeout(500) // Wait for mock data
 
-  await expect(page.getByText('Laputa')).toBeVisible()
-  await expect(page.getByText('All Notes')).toBeVisible()
+  await expect(page.getByRole('heading', { name: 'Laputa' })).toBeVisible()
+  await expect(page.getByText('PROJECTS')).toBeVisible()
+  await expect(page.getByText('EXPERIMENTS')).toBeVisible()
+  await expect(page.getByText('RESPONSIBILITIES')).toBeVisible()
+  await expect(page.getByText('PROCEDURES')).toBeVisible()
 })
diff --git a/src/App.tsx b/src/App.tsx
index d8f3f901..fd360e53 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -6,14 +6,17 @@ import { Editor } from './components/Editor'
 import { Inspector } from './components/Inspector'
 import { ResizeHandle } from './components/ResizeHandle'
 import { isTauri, mockInvoke } from './mock-tauri'
-import type { VaultEntry } from './types'
+import type { VaultEntry, SidebarSelection } from './types'
 import './App.css'
 
 // TODO: Make vault path configurable via settings
 const TEST_VAULT_PATH = '~/Laputa'
 
+const DEFAULT_SELECTION: SidebarSelection = { kind: 'filter', filter: 'all' }
+
 function App() {
   const [entries, setEntries] = useState<VaultEntry[]>([])
+  const [selection, setSelection] = useState<SidebarSelection>(DEFAULT_SELECTION)
   const [sidebarWidth, setSidebarWidth] = useState(250)
   const [noteListWidth, setNoteListWidth] = useState(300)
   const [inspectorWidth, setInspectorWidth] = useState(280)
@@ -56,7 +59,7 @@ function App() {
   return (
     <div className="app">
       <div className="app__sidebar" style={{ width: sidebarWidth }}>
-        <Sidebar />
+        <Sidebar entries={entries} selection={selection} onSelect={setSelection} />
       </div>
       <ResizeHandle onResize={handleSidebarResize} />
       <div className="app__note-list" style={{ width: noteListWidth }}>
diff --git a/src/components/Sidebar.css b/src/components/Sidebar.css
index 83039114..83634ff5 100644
--- a/src/components/Sidebar.css
+++ b/src/components/Sidebar.css
@@ -4,11 +4,13 @@
   display: flex;
   flex-direction: column;
   overflow-y: auto;
+  height: 100%;
 }
 
 .sidebar__header {
   padding: 16px;
   border-bottom: 1px solid #2a2a4a;
+  flex-shrink: 0;
 }
 
 .sidebar__header h2 {
@@ -20,14 +22,97 @@
 
 .sidebar__nav {
   padding: 8px 0;
+  flex: 1;
+  overflow-y: auto;
+}
+
+/* Section Groups */
+.sidebar__section {
+  margin-bottom: 4px;
+}
+
+.sidebar__section-header {
+  display: flex;
+  align-items: center;
+  padding: 6px 8px;
+  margin: 0 4px;
+  border-radius: 4px;
+  cursor: pointer;
+  font-size: 11px;
+  font-weight: 600;
+  letter-spacing: 0.05em;
+  color: #888;
+  user-select: none;
+}
+
+.sidebar__section-header:hover {
+  background: #2a2a4a;
+  color: #bbb;
+}
+
+.sidebar__section-header--active {
+  background: #2a2a4a;
+  color: #ffffff;
+}
+
+.sidebar__collapse-btn {
+  background: none;
+  border: none;
+  color: inherit;
+  font-size: 10px;
+  cursor: pointer;
+  padding: 0 4px 0 0;
+  line-height: 1;
+  width: 16px;
+  flex-shrink: 0;
+}
+
+.sidebar__section-label {
+  flex: 1;
+}
+
+.sidebar__section-count {
+  font-size: 10px;
+  color: #666;
+  margin-right: 4px;
+}
+
+.sidebar__add-btn {
+  background: none;
+  border: none;
+  color: #666;
+  font-size: 14px;
+  cursor: pointer;
+  padding: 0 2px;
+  line-height: 1;
+  border-radius: 3px;
+  opacity: 0;
+  transition: opacity 0.15s;
+}
+
+.sidebar__section-header:hover .sidebar__add-btn {
+  opacity: 1;
+}
+
+.sidebar__add-btn:hover {
+  color: #ffffff;
+  background: #3a3a5a;
+}
+
+/* Section items (entity list) */
+.sidebar__section-items {
+  padding: 2px 0;
 }
 
 .sidebar__item {
-  padding: 8px 16px;
+  padding: 5px 16px 5px 28px;
   cursor: pointer;
-  font-size: 14px;
+  font-size: 13px;
   border-radius: 4px;
-  margin: 2px 8px;
+  margin: 1px 4px;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
 }
 
 .sidebar__item:hover {
diff --git a/src/components/Sidebar.test.tsx b/src/components/Sidebar.test.tsx
index b6739c1f..1fca7f6d 100644
--- a/src/components/Sidebar.test.tsx
+++ b/src/components/Sidebar.test.tsx
@@ -1,18 +1,128 @@
-import { render, screen } from '@testing-library/react'
-import { describe, it, expect } from 'vitest'
+import { render, screen, fireEvent } from '@testing-library/react'
+import { describe, it, expect, vi } from 'vitest'
 import { Sidebar } from './Sidebar'
+import type { VaultEntry, SidebarSelection } from '../types'
+
+const mockEntries: VaultEntry[] = [
+  {
+    path: '/vault/project/build-app.md',
+    filename: 'build-app.md',
+    title: 'Build Laputa App',
+    isA: 'Project',
+    aliases: [],
+    belongsTo: [],
+    relatedTo: [],
+    status: 'Active',
+    owner: 'Luca',
+    cadence: null,
+    modifiedAt: 1700000000,
+    fileSize: 1024,
+  },
+  {
+    path: '/vault/responsibility/grow-newsletter.md',
+    filename: 'grow-newsletter.md',
+    title: 'Grow Newsletter',
+    isA: 'Responsibility',
+    aliases: [],
+    belongsTo: [],
+    relatedTo: [],
+    status: 'Active',
+    owner: 'Luca',
+    cadence: null,
+    modifiedAt: 1700000000,
+    fileSize: 512,
+  },
+  {
+    path: '/vault/experiment/stock-screener.md',
+    filename: 'stock-screener.md',
+    title: 'Stock Screener',
+    isA: 'Experiment',
+    aliases: [],
+    belongsTo: [],
+    relatedTo: [],
+    status: 'Active',
+    owner: 'Luca',
+    cadence: null,
+    modifiedAt: 1700000000,
+    fileSize: 256,
+  },
+  {
+    path: '/vault/procedure/weekly-essays.md',
+    filename: 'weekly-essays.md',
+    title: 'Write Weekly Essays',
+    isA: 'Procedure',
+    aliases: [],
+    belongsTo: [],
+    relatedTo: [],
+    status: 'Active',
+    owner: 'Luca',
+    cadence: 'Weekly',
+    modifiedAt: 1700000000,
+    fileSize: 128,
+  },
+]
+
+const defaultSelection: SidebarSelection = { kind: 'filter', filter: 'all' }
 
 describe('Sidebar', () => {
   it('renders the app title', () => {
-    render(<Sidebar />)
+    render(<Sidebar entries={[]} selection={defaultSelection} onSelect={() => {}} />)
     expect(screen.getByText('Laputa')).toBeInTheDocument()
   })
 
-  it('renders navigation items', () => {
-    render(<Sidebar />)
-    expect(screen.getByText('All Notes')).toBeInTheDocument()
-    expect(screen.getByText('Projects')).toBeInTheDocument()
-    expect(screen.getByText('Tasks')).toBeInTheDocument()
-    expect(screen.getByText('People')).toBeInTheDocument()
+  it('renders section group headers', () => {
+    render(<Sidebar entries={mockEntries} selection={defaultSelection} onSelect={() => {}} />)
+    expect(screen.getByText('PROJECTS')).toBeInTheDocument()
+    expect(screen.getByText('EXPERIMENTS')).toBeInTheDocument()
+    expect(screen.getByText('RESPONSIBILITIES')).toBeInTheDocument()
+    expect(screen.getByText('PROCEDURES')).toBeInTheDocument()
+  })
+
+  it('shows entity names under their section groups', () => {
+    render(<Sidebar entries={mockEntries} selection={defaultSelection} onSelect={() => {}} />)
+    expect(screen.getByText('Build Laputa App')).toBeInTheDocument()
+    expect(screen.getByText('Grow Newsletter')).toBeInTheDocument()
+    expect(screen.getByText('Stock Screener')).toBeInTheDocument()
+    expect(screen.getByText('Write Weekly Essays')).toBeInTheDocument()
+  })
+
+  it('shows correct counts per section', () => {
+    render(<Sidebar entries={mockEntries} selection={defaultSelection} onSelect={() => {}} />)
+    // Each section has exactly 1 item
+    const counts = screen.getAllByText('1')
+    expect(counts.length).toBe(4)
+  })
+
+  it('collapses and expands sections', () => {
+    render(<Sidebar entries={mockEntries} selection={defaultSelection} onSelect={() => {}} />)
+    expect(screen.getByText('Build Laputa App')).toBeInTheDocument()
+
+    // Collapse PROJECTS section
+    fireEvent.click(screen.getByLabelText('Collapse PROJECTS'))
+    expect(screen.queryByText('Build Laputa App')).not.toBeInTheDocument()
+
+    // Expand PROJECTS section
+    fireEvent.click(screen.getByLabelText('Expand PROJECTS'))
+    expect(screen.getByText('Build Laputa App')).toBeInTheDocument()
+  })
+
+  it('calls onSelect when clicking an entity', () => {
+    const onSelect = vi.fn()
+    render(<Sidebar entries={mockEntries} selection={defaultSelection} onSelect={onSelect} />)
+    fireEvent.click(screen.getByText('Build Laputa App'))
+    expect(onSelect).toHaveBeenCalledWith({
+      kind: 'entity',
+      entry: mockEntries[0],
+    })
+  })
+
+  it('calls onSelect when clicking a section header', () => {
+    const onSelect = vi.fn()
+    render(<Sidebar entries={mockEntries} selection={defaultSelection} onSelect={onSelect} />)
+    fireEvent.click(screen.getByText('PROJECTS'))
+    expect(onSelect).toHaveBeenCalledWith({
+      kind: 'sectionGroup',
+      type: 'Project',
+    })
   })
 })
diff --git a/src/components/Sidebar.tsx b/src/components/Sidebar.tsx
index 8781ee86..c33008f4 100644
--- a/src/components/Sidebar.tsx
+++ b/src/components/Sidebar.tsx
@@ -1,16 +1,96 @@
+import { useState } from 'react'
+import type { VaultEntry, SidebarSelection } from '../types'
 import './Sidebar.css'
 
-export function Sidebar() {
+interface SidebarProps {
+  entries: VaultEntry[]
+  selection: SidebarSelection
+  onSelect: (selection: SidebarSelection) => void
+}
+
+const SECTION_GROUPS = [
+  { label: 'PROJECTS', type: 'Project' },
+  { label: 'EXPERIMENTS', type: 'Experiment' },
+  { label: 'RESPONSIBILITIES', type: 'Responsibility' },
+  { label: 'PROCEDURES', type: 'Procedure' },
+] as const
+
+export function Sidebar({ entries, selection, onSelect }: SidebarProps) {
+  const [collapsed, setCollapsed] = useState<Record<string, boolean>>({})
+
+  const toggleSection = (type: string) => {
+    setCollapsed((prev) => ({ ...prev, [type]: !prev[type] }))
+  }
+
+  const isActive = (sel: SidebarSelection): boolean => {
+    if (selection.kind !== sel.kind) return false
+    if (sel.kind === 'filter' && selection.kind === 'filter') return sel.filter === selection.filter
+    if (sel.kind === 'sectionGroup' && selection.kind === 'sectionGroup') return sel.type === selection.type
+    if (sel.kind === 'entity' && selection.kind === 'entity') return sel.entry.path === selection.entry.path
+    if (sel.kind === 'topic' && selection.kind === 'topic') return sel.entry.path === selection.entry.path
+    return false
+  }
+
   return (
     <aside className="sidebar">
       <div className="sidebar__header">
         <h2>Laputa</h2>
       </div>
+
       <nav className="sidebar__nav">
-        <div className="sidebar__item sidebar__item--active">All Notes</div>
-        <div className="sidebar__item">Projects</div>
-        <div className="sidebar__item">Tasks</div>
-        <div className="sidebar__item">People</div>
+        {SECTION_GROUPS.map(({ label, type }) => {
+          const items = entries.filter((e) => e.isA === type)
+          const isCollapsed = collapsed[type] ?? false
+
+          return (
+            <div key={type} className="sidebar__section">
+              <div
+                className={`sidebar__section-header${
+                  isActive({ kind: 'sectionGroup', type }) ? ' sidebar__section-header--active' : ''
+                }`}
+                onClick={() => onSelect({ kind: 'sectionGroup', type })}
+              >
+                <button
+                  className="sidebar__collapse-btn"
+                  onClick={(e) => {
+                    e.stopPropagation()
+                    toggleSection(type)
+                  }}
+                  aria-label={isCollapsed ? `Expand ${label}` : `Collapse ${label}`}
+                >
+                  {isCollapsed ? '▸' : '▾'}
+                </button>
+                <span className="sidebar__section-label">{label}</span>
+                <span className="sidebar__section-count">{items.length}</span>
+                <button
+                  className="sidebar__add-btn"
+                  onClick={(e) => {
+                    e.stopPropagation()
+                    // TODO: Wire up create new entity
+                  }}
+                  aria-label={`Add ${type}`}
+                >
+                  +
+                </button>
+              </div>
+              {!isCollapsed && (
+                <div className="sidebar__section-items">
+                  {items.map((entry) => (
+                    <div
+                      key={entry.path}
+                      className={`sidebar__item${
+                        isActive({ kind: 'entity', entry }) ? ' sidebar__item--active' : ''
+                      }`}
+                      onClick={() => onSelect({ kind: 'entity', entry })}
+                    >
+                      {entry.title}
+                    </div>
+                  ))}
+                </div>
+              )}
+            </div>
+          )
+        })}
       </nav>
     </aside>
   )
diff --git a/src/types.ts b/src/types.ts
index b82469c1..bce1a16f 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -12,3 +12,9 @@ export interface VaultEntry {
   modifiedAt: number | null
   fileSize: number
 }
+
+export type SidebarSelection =
+  | { kind: 'filter'; filter: 'all' | 'people' | 'events' | 'favorites' | 'trash' }
+  | { kind: 'sectionGroup'; type: string }
+  | { kind: 'entity'; entry: VaultEntry }
+  | { kind: 'topic'; entry: VaultEntry }