diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index 27b199b..5968fc0 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -1,7 +1,8 @@
 import { Routes, Route, Navigate } from 'react-router-dom'
 import { CssBaseline, ThemeProvider } from '@mui/material'
-import { lightTheme } from './theme/theme'
+import { darkTheme } from './theme/theme'
 import { useAuthStore } from './store/authStore'
+import AppLayout from './components/AppLayout'
 import LoginPage from './pages/LoginPage'
 import MfaSetupPage from './pages/MfaSetupPage'
 import HostsPage from './pages/HostsPage'
@@ -16,14 +17,6 @@ import CertificatesPage from './pages/CertificatesPage'
 import ReportsPage from './pages/ReportsPage'
 import SettingsPage from './pages/SettingsPage'
 
-// Placeholder pages — implemented in later milestones
-const PlaceholderPage = ({ title }: { title: string }) => (
-  <div style={{ padding: 32 }}>
-    <h2>{title}</h2>
-    <p>Coming soon in a future milestone.</p>
-  </div>
-)
-
 function RequireAuth({ children }: { children: React.ReactNode }) {
   const isAuthenticated = useAuthStore((s) => s.isAuthenticated)
   return isAuthenticated ? <>{children}</> : <Navigate to="/login" replace />
@@ -31,38 +24,30 @@ function RequireAuth({ children }: { children: React.ReactNode }) {
 
 function App() {
   return (
-    <ThemeProvider theme={lightTheme}>
+    <ThemeProvider theme={darkTheme}>
       <CssBaseline />
       <Routes>
         {/* Public */}
         <Route path="/login" element={<LoginPage />} />
 
-        {/* Protected — M2 */}
-        <Route path="/" element={<RequireAuth><Navigate to="/dashboard" replace /></RequireAuth>} />
-        <Route path="/mfa/setup" element={<RequireAuth><MfaSetupPage /></RequireAuth>} />
+        {/* Protected — wrapped in AppLayout with sidebar navigation */}
+        <Route element={<RequireAuth><AppLayout /></RequireAuth>}>
+          <Route path="/" element={<Navigate to="/dashboard" replace />} />
+          <Route path="/mfa/setup" element={<MfaSetupPage />} />
+          <Route path="/dashboard" element={<DashboardPage />} />
+          <Route path="/hosts" element={<HostsPage />} />
+          <Route path="/hosts/:id" element={<HostDetailPage />} />
+          <Route path="/groups" element={<GroupsPage />} />
+          <Route path="/users" element={<UsersPage />} />
+          <Route path="/jobs" element={<JobsPage />} />
+          <Route path="/deployment" element={<PatchDeploymentPage />} />
+          <Route path="/maintenance" element={<MaintenanceWindowsPage />} />
+          <Route path="/reports" element={<ReportsPage />} />
+          <Route path="/certificates" element={<CertificatesPage />} />
+          <Route path="/settings" element={<SettingsPage />} />
+        </Route>
 
-        {/* Protected — M3 */}
-        <Route path="/dashboard" element={<RequireAuth><DashboardPage /></RequireAuth>} />
-        <Route path="/hosts" element={<RequireAuth><HostsPage /></RequireAuth>} />
-        <Route path="/hosts/:id" element={<RequireAuth><HostDetailPage /></RequireAuth>} />
-        <Route path="/groups" element={<RequireAuth><GroupsPage /></RequireAuth>} />
-        <Route path="/users" element={<RequireAuth><UsersPage /></RequireAuth>} />
-
-        {/* Protected — M5 */}
-        <Route path="/jobs" element={<RequireAuth><JobsPage /></RequireAuth>} />
-        <Route path="/deployment" element={<RequireAuth><PatchDeploymentPage /></RequireAuth>} />
-
-        {/* Protected — M6 */}
-        <Route path="/maintenance" element={<RequireAuth><MaintenanceWindowsPage /></RequireAuth>} />
-
-        {/* Placeholder — later milestones */}
-        {/* Protected — M9 */}
-        <Route path="/reports" element={<RequireAuth><ReportsPage /></RequireAuth>} />
-        {/* Protected — M8 */}
-        <Route path="/certificates" element={<RequireAuth><CertificatesPage /></RequireAuth>} />
-        <Route path="/settings" element={<RequireAuth><SettingsPage /></RequireAuth>} />
-
-        <Route path="*" element={<PlaceholderPage title="404 Not Found" />} />
+        <Route path="*" element={<Navigate to="/dashboard" replace />} />
       </Routes>
     </ThemeProvider>
   )
diff --git a/frontend/src/components/AppLayout.tsx b/frontend/src/components/AppLayout.tsx
new file mode 100644
index 0000000..e98fa31
--- /dev/null
+++ b/frontend/src/components/AppLayout.tsx
@@ -0,0 +1,229 @@
+import { useState } from 'react'
+import { Outlet, useNavigate, useLocation } from 'react-router-dom'
+import {
+  AppBar, Box, CssBaseline, Divider, Drawer, IconButton,
+  List, ListItem, ListItemButton, ListItemIcon, ListItemText,
+  Toolbar, Typography, Avatar, Menu, MenuItem, Tooltip,
+} from '@mui/material'
+import {
+  Dashboard as DashboardIcon,
+  Computer as HostsIcon,
+  Group as GroupsIcon,
+  Build as DeployIcon,
+  Assignment as JobsIcon,
+  Schedule as MaintenanceIcon,
+  People as UsersIcon,
+  VerifiedUser as CertsIcon,
+  Assessment as ReportsIcon,
+  Settings as SettingsIcon,
+  Menu as MenuIcon,
+  Logout as LogoutIcon,
+  Person as PersonIcon,
+} from '@mui/icons-material'
+import { useAuthStore } from '../store/authStore'
+
+const DRAWER_WIDTH = 240
+
+interface NavItem {
+  label: string
+  path: string
+  icon: React.ReactElement
+  adminOnly?: boolean
+}
+
+const navGroups: { heading: string; items: NavItem[] }[] = [
+  {
+    heading: 'Overview',
+    items: [
+      { label: 'Dashboard', path: '/dashboard', icon: <DashboardIcon /> },
+    ],
+  },
+  {
+    heading: 'Fleet',
+    items: [
+      { label: 'Hosts', path: '/hosts', icon: <HostsIcon /> },
+      { label: 'Groups', path: '/groups', icon: <GroupsIcon /> },
+      { label: 'Deploy', path: '/deployment', icon: <DeployIcon /> },
+    ],
+  },
+  {
+    heading: 'Operations',
+    items: [
+      { label: 'Jobs', path: '/jobs', icon: <JobsIcon /> },
+      { label: 'Maintenance', path: '/maintenance', icon: <MaintenanceIcon /> },
+    ],
+  },
+  {
+    heading: 'Administration',
+    items: [
+      { label: 'Users', path: '/users', icon: <UsersIcon />, adminOnly: true },
+      { label: 'Certificates', path: '/certificates', icon: <CertsIcon />, adminOnly: true },
+      { label: 'Reports', path: '/reports', icon: <ReportsIcon /> },
+      { label: 'Settings', path: '/settings', icon: <SettingsIcon />, adminOnly: true },
+    ],
+  },
+]
+
+export default function AppLayout() {
+  const navigate = useNavigate()
+  const location = useLocation()
+  const { user, logout } = useAuthStore()
+  const [mobileOpen, setMobileOpen] = useState(false)
+  const [anchorEl, setAnchorEl] = useState<null | HTMLElement>(null)
+
+  const isAdmin = user?.role === 'admin'
+
+  const handleDrawerToggle = () => setMobileOpen(!mobileOpen)
+  const handleMenuOpen = (e: React.MouseEvent<HTMLElement>) => setAnchorEl(e.currentTarget)
+  const handleMenuClose = () => setAnchorEl(null)
+
+  const handleLogout = () => {
+    handleMenuClose()
+    logout()
+    navigate('/login', { replace: true })
+  }
+
+  const drawer = (
+    <Box sx={{ height: '100%', display: 'flex', flexDirection: 'column' }}>
+      <Toolbar sx={{ justifyContent: 'center', py: 1.5 }}>
+        <Typography variant="h6" fontWeight={700} sx={{
+          background: 'linear-gradient(135deg, #42A5F5 30%, #26C6DA 100%)',
+          WebkitBackgroundClip: 'text',
+          WebkitTextFillColor: 'transparent',
+        }}>
+          🐉 Patch Manager
+        </Typography>
+      </Toolbar>
+      <Divider />
+      <Box sx={{ flex: 1, overflowY: 'auto', py: 1 }}>
+        {navGroups.map((group) => {
+          const visibleItems = group.items.filter((item) => !item.adminOnly || isAdmin)
+          if (visibleItems.length === 0) return null
+          return (
+            <Box key={group.heading} sx={{ mb: 1 }}>
+              <Typography variant="caption" color="text.secondary" sx={{ px: 2.5, py: 0.5, fontWeight: 600, textTransform: 'uppercase', letterSpacing: 0.5 }}>
+                {group.heading}
+              </Typography>
+              <List dense disablePadding>
+                {visibleItems.map((item) => {
+                  const isActive = location.pathname === item.path || location.pathname.startsWith(item.path + '/')
+                  return (
+                    <ListItem key={item.path} disablePadding sx={{ px: 1 }}>
+                      <ListItemButton
+                        selected={isActive}
+                        onClick={() => navigate(item.path)}
+                        sx={{
+                          borderRadius: 1,
+                          mx: 0.5,
+                          '&.Mui-selected': {
+                            bgcolor: 'primary.main',
+                            color: 'primary.contrastText',
+                            '&:hover': { bgcolor: 'primary.dark' },
+                            '& .MuiListItemIcon-root': { color: 'primary.contrastText' },
+                          },
+                        }}
+                      >
+                        <ListItemIcon sx={{ minWidth: 36, color: isActive ? 'inherit' : 'text.secondary' }}>
+                          {item.icon}
+                        </ListItemIcon>
+                        <ListItemText primary={item.label} primaryTypographyProps={{ fontWeight: isActive ? 600 : 400, fontSize: '0.875rem' }} />
+                      </ListItemButton>
+                    </ListItem>
+                  )
+                })}
+              </List>
+            </Box>
+          )
+        })}
+      </Box>
+      <Divider />
+      <Box sx={{ p: 1.5 }}>
+        <Typography variant="caption" color="text.secondary">
+          Linux Patch Manager v0.1.0
+        </Typography>
+      </Box>
+    </Box>
+  )
+
+  return (
+    <Box sx={{ display: 'flex', height: '100vh' }}>
+      <CssBaseline />
+
+      {/* App Bar */}
+      <AppBar
+        position="fixed"
+        elevation={0}
+        sx={{
+          zIndex: (theme) => theme.zIndex.drawer + 1,
+          borderBottom: 1,
+          borderColor: 'divider',
+        }}
+      >
+        <Toolbar>
+          <IconButton
+            color="inherit"
+            edge="start"
+            onClick={handleDrawerToggle}
+            sx={{ mr: 2, display: { md: 'none' } }}
+          >
+            <MenuIcon />
+          </IconButton>
+          <Typography variant="h6" noWrap sx={{ flexGrow: 1, fontWeight: 600 }}>
+            {navGroups.flatMap((g) => g.items).find((i) => location.pathname === i.path || location.pathname.startsWith(i.path + '/'))?.label || 'Patch Manager'}
+          </Typography>
+          <Tooltip title={`${user?.display_name || user?.username} (${user?.role})`}>
+            <IconButton onClick={handleMenuOpen} color="inherit" sx={{ ml: 1 }}>
+              <Avatar sx={{ width: 32, height: 32, bgcolor: 'secondary.main', fontSize: '0.875rem' }}>
+                {(user?.display_name || user?.username || '?')[0].toUpperCase()}
+              </Avatar>
+            </IconButton>
+          </Tooltip>
+          <Menu
+            anchorEl={anchorEl}
+            open={Boolean(anchorEl)}
+            onClose={handleMenuClose}
+            slotProps={{ paper: { sx: { mt: 1 } } }}
+          >
+            <MenuItem disabled>
+              <ListItemIcon><PersonIcon fontSize="small" /></ListItemIcon>
+              <ListItemText primary={user?.display_name || user?.username} secondary={user?.role} />
+            </MenuItem>
+            <Divider />
+            <MenuItem onClick={handleLogout}>
+              <ListItemIcon><LogoutIcon fontSize="small" /></ListItemIcon>
+              <ListItemText primary="Sign out" />
+            </MenuItem>
+          </Menu>
+        </Toolbar>
+      </AppBar>
+
+      {/* Sidebar */}
+      <Box component="nav" sx={{ width: { md: DRAWER_WIDTH }, flexShrink: { md: 0 } }}>
+        {/* Mobile drawer */}
+        <Drawer
+          variant="temporary"
+          open={mobileOpen}
+          onClose={handleDrawerToggle}
+          ModalProps={{ keepMounted: true }}
+          sx={{ display: { xs: 'block', md: 'none' }, '& .MuiDrawer-paper': { boxSizing: 'border-box', width: DRAWER_WIDTH } }}
+        >
+          {drawer}
+        </Drawer>
+        {/* Desktop drawer */}
+        <Drawer
+          variant="permanent"
+          sx={{ display: { xs: 'none', md: 'block' }, '& .MuiDrawer-paper': { boxSizing: 'border-box', width: DRAWER_WIDTH } }}
+          open
+        >
+          {drawer}
+        </Drawer>
+      </Box>
+
+      {/* Main content */}
+      <Box component="main" sx={{ flexGrow: 1, p: 3, bgcolor: 'background.default', overflowY: 'auto' }}>
+        <Toolbar />
+        <Outlet />
+      </Box>
+    </Box>
+  )
+}