diff --git a/apps/worker/src/api/sessions.ts b/apps/worker/src/api/sessions.ts
new file mode 100644
index 0000000..127ce91
--- /dev/null
+++ b/apps/worker/src/api/sessions.ts
@@ -0,0 +1,46 @@
+import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
+import type { StartSessionInput, WorkSession } from '@solelog/shared';
+import { apiFetch } from '../lib/api';
+
+export function useActiveSessions() {
+  return useQuery({
+    queryKey: ['sessions', 'active'],
+    queryFn: () => apiFetch<WorkSession[]>('/api/sessions/active'),
+  });
+}
+
+export function useStartSession() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (input: StartSessionInput) =>
+      apiFetch<WorkSession>('/api/sessions/start', {
+        method: 'POST',
+        body: JSON.stringify(input),
+      }),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['sessions'] });
+    },
+  });
+}
+
+export function useStopSession() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (id: number) =>
+      apiFetch<WorkSession>(`/api/sessions/${id}/stop`, { method: 'POST' }),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['sessions'] });
+    },
+  });
+}
+
+export function useDiscardSession() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (id: number) =>
+      apiFetch<WorkSession>(`/api/sessions/${id}/discard`, { method: 'POST' }),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['sessions'] });
+    },
+  });
+}
diff --git a/apps/worker/src/lib/stopwatch.test.ts b/apps/worker/src/lib/stopwatch.test.ts
new file mode 100644
index 0000000..c145929
--- /dev/null
+++ b/apps/worker/src/lib/stopwatch.test.ts
@@ -0,0 +1,19 @@
+import { describe, expect, it } from 'vitest';
+import { elapsedSeconds, formatTime } from './stopwatch';
+
+describe('stopwatch logic', () => {
+  it('formats seconds as HH:MM:SS', () => {
+    expect(formatTime(0)).toBe('00:00:00');
+    expect(formatTime(65)).toBe('00:01:05');
+    expect(formatTime(3661)).toBe('01:01:01');
+  });
+
+  it('computes elapsed seconds since start excluding paused time', () => {
+    expect(elapsedSeconds(1000, 6000, 0)).toBe(5);
+    expect(elapsedSeconds(1000, 6000, 2000)).toBe(3);
+  });
+
+  it('never returns a negative elapsed value', () => {
+    expect(elapsedSeconds(5000, 1000, 0)).toBe(0);
+  });
+});
diff --git a/apps/worker/src/lib/stopwatch.ts b/apps/worker/src/lib/stopwatch.ts
new file mode 100644
index 0000000..0f1d094
--- /dev/null
+++ b/apps/worker/src/lib/stopwatch.ts
@@ -0,0 +1,16 @@
+// Pure timing helpers for the Stopwatch screen — server-authoritative elapsed.
+// Elapsed is computed from the server start_time (wall-clock), not a tick counter,
+// so it survives backgrounding. Pause accumulates paused time client-side.
+
+export function formatTime(totalSeconds: number): string {
+  const s = Math.max(0, Math.floor(totalSeconds));
+  const h = Math.floor(s / 3600);
+  const m = Math.floor((s % 3600) / 60);
+  const sec = s % 60;
+  return `${String(h).padStart(2, '0')}:${String(m).padStart(2, '0')}:${String(sec).padStart(2, '0')}`;
+}
+
+// elapsed seconds since startMs, excluding accumulated paused ms, evaluated at nowMs.
+export function elapsedSeconds(startMs: number, nowMs: number, pausedMs: number): number {
+  return Math.max(0, Math.floor((nowMs - startMs - pausedMs) / 1000));
+}
diff --git a/apps/worker/src/screens/Stopwatch.test.tsx b/apps/worker/src/screens/Stopwatch.test.tsx
new file mode 100644
index 0000000..9f085a3
--- /dev/null
+++ b/apps/worker/src/screens/Stopwatch.test.tsx
@@ -0,0 +1,185 @@
+import { render, screen, waitFor } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import type { Activity, WorkSession } from '@solelog/shared';
+import Stopwatch from './Stopwatch';
+import { useActivities } from '../api/activities';
+import {
+  useActiveSessions,
+  useStartSession,
+  useStopSession,
+  useDiscardSession,
+} from '../api/sessions';
+
+vi.mock('../api/activities', () => ({
+  useActivities: vi.fn(),
+}));
+
+vi.mock('../api/sessions', () => ({
+  useActiveSessions: vi.fn(),
+  useStartSession: vi.fn(),
+  useStopSession: vi.fn(),
+  useDiscardSession: vi.fn(),
+}));
+
+const mockedUseActivities = vi.mocked(useActivities);
+const mockedUseActiveSessions = vi.mocked(useActiveSessions);
+const mockedUseStartSession = vi.mocked(useStartSession);
+const mockedUseStopSession = vi.mocked(useStopSession);
+const mockedUseDiscardSession = vi.mocked(useDiscardSession);
+
+const FREZEN: Activity = {
+  id: 1,
+  name: 'Frezen',
+  insole_types: ['Kurk', 'Berk'],
+  created_at: '2026-01-01T00:00:00.000Z',
+};
+const PRINTEN: Activity = {
+  id: 2,
+  name: 'Printen',
+  insole_types: ['3D'],
+  created_at: '2026-01-01T00:00:00.000Z',
+};
+
+function activeSession(overrides: Partial<WorkSession> = {}): WorkSession {
+  return {
+    id: 99,
+    user_id: 'u1',
+    activity_id: 1,
+    activity_name: 'Frezen',
+    insole_type: 'Kurk',
+    pair_count: 2,
+    start_time: new Date().toISOString(),
+    end_time: null,
+    duration_seconds: null,
+    status: 'active',
+    source: 'app',
+    notes: null,
+    created_at: new Date().toISOString(),
+    ...overrides,
+  };
+}
+
+// Build a fake RQ query/mutation result object the screen can consume.
+function query<R>(data: unknown): R {
+  return { data, isLoading: false, isError: false } as unknown as R;
+}
+
+let startMutate: ReturnType<typeof vi.fn>;
+let stopMutate: ReturnType<typeof vi.fn>;
+let discardMutate: ReturnType<typeof vi.fn>;
+
+function mutation<R>(mutate: ReturnType<typeof vi.fn>): R {
+  return { mutate, isPending: false } as unknown as R;
+}
+
+function renderStopwatch() {
+  const queryClient = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  return render(
+    <QueryClientProvider client={queryClient}>
+      <Stopwatch />
+    </QueryClientProvider>,
+  );
+}
+
+describe('Stopwatch', () => {
+  beforeEach(() => {
+    startMutate = vi.fn();
+    stopMutate = vi.fn();
+    discardMutate = vi.fn();
+    mockedUseActivities.mockReturnValue(query<ReturnType<typeof useActivities>>([FREZEN, PRINTEN]));
+    mockedUseActiveSessions.mockReturnValue(query<ReturnType<typeof useActiveSessions>>([]));
+    mockedUseStartSession.mockReturnValue(mutation<ReturnType<typeof useStartSession>>(startMutate));
+    mockedUseStopSession.mockReturnValue(mutation<ReturnType<typeof useStopSession>>(stopMutate));
+    mockedUseDiscardSession.mockReturnValue(
+      mutation<ReturnType<typeof useDiscardSession>>(discardMutate),
+    );
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+    vi.useRealTimers();
+  });
+
+  it('renders the three sections and Start button in Dutch', () => {
+    renderStopwatch();
+    expect(screen.getByText('Type zool')).toBeInTheDocument();
+    expect(screen.getByText('Type handeling')).toBeInTheDocument();
+    expect(screen.getByText('Aantal zolen')).toBeInTheDocument();
+    expect(screen.getByRole('button', { name: 'Start Stopwatch' })).toBeInTheDocument();
+    expect(screen.getByRole('button', { name: 'Kurk' })).toBeInTheDocument();
+    expect(screen.getByRole('button', { name: 'Berk' })).toBeInTheDocument();
+    expect(screen.getByRole('button', { name: '3D' })).toBeInTheDocument();
+    // Default count is 2 (free-typed text mirror, so the value is the string '2').
+    expect(screen.getByLabelText('Aantal zolen')).toHaveValue('2');
+  });
+
+  it('disables Start until a handling is chosen', async () => {
+    const user = userEvent.setup();
+    renderStopwatch();
+
+    const startBtn = screen.getByRole('button', { name: 'Start Stopwatch' });
+    expect(startBtn).toBeDisabled();
+
+    // Kurk is the default zooltype; pick the Frezen handling.
+    await user.selectOptions(screen.getByLabelText('Type handeling'), 'Frezen');
+    await waitFor(() => expect(startBtn).toBeEnabled());
+  });
+
+  it('filters handlings by the chosen zooltype', async () => {
+    const user = userEvent.setup();
+    renderStopwatch();
+
+    const select = screen.getByLabelText('Type handeling');
+    // Kurk selected by default -> Frezen present, Printen absent.
+    expect(screen.getByRole('option', { name: 'Frezen' })).toBeInTheDocument();
+    expect(screen.queryByRole('option', { name: 'Printen' })).not.toBeInTheDocument();
+
+    // Switch to 3D -> Printen present, Frezen absent.
+    await user.click(screen.getByRole('button', { name: '3D' }));
+    expect(screen.getByRole('option', { name: 'Printen' })).toBeInTheDocument();
+    expect(screen.queryByRole('option', { name: 'Frezen' })).not.toBeInTheDocument();
+    expect(select).toBeInTheDocument();
+  });
+
+  it('calls start with the selected values', async () => {
+    const user = userEvent.setup();
+    renderStopwatch();
+
+    // Pick Berk; Frezen applies to Berk.
+    await user.click(screen.getByRole('button', { name: 'Berk' }));
+    await user.selectOptions(screen.getByLabelText('Type handeling'), 'Frezen');
+    // Bump count to 3.
+    await user.click(screen.getByRole('button', { name: 'Meer zolen' }));
+
+    await user.click(screen.getByRole('button', { name: 'Start Stopwatch' }));
+
+    expect(startMutate).toHaveBeenCalledTimes(1);
+    expect(startMutate.mock.calls[0][0]).toMatchObject({
+      activity_id: 1,
+      insole_type: 'Berk',
+      pair_count: 3,
+    });
+  });
+
+  it('arms discard on first Annuleren tap and discards on the second', async () => {
+    const user = userEvent.setup();
+    // Recover an active session so the screen renders the running UI.
+    mockedUseActiveSessions.mockReturnValue(
+      query<ReturnType<typeof useActiveSessions>>([activeSession()]),
+    );
+    renderStopwatch();
+
+    const cancel = await screen.findByRole('button', { name: 'Annuleren' });
+    await user.click(cancel);
+
+    expect(
+      screen.getByRole('button', { name: 'Nogmaals tikken ter bevestiging' }),
+    ).toBeInTheDocument();
+
+    await user.click(screen.getByRole('button', { name: 'Nogmaals tikken ter bevestiging' }));
+    expect(discardMutate).toHaveBeenCalledTimes(1);
+    expect(discardMutate.mock.calls[0][0]).toBe(99);
+  });
+});
diff --git a/apps/worker/src/screens/Stopwatch.tsx b/apps/worker/src/screens/Stopwatch.tsx
index 8e1d718..498a141 100644
--- a/apps/worker/src/screens/Stopwatch.tsx
+++ b/apps/worker/src/screens/Stopwatch.tsx
@@ -1,7 +1,366 @@
+import { useEffect, useRef, useState } from 'react';
+import type { InsoleType, WorkSession } from '@solelog/shared';
+import { useActivities } from '../api/activities';
+import {
+  useActiveSessions,
+  useStartSession,
+  useStopSession,
+  useDiscardSession,
+} from '../api/sessions';
+import { elapsedSeconds, formatTime } from '../lib/stopwatch';
+
+const ALL_TYPES: InsoleType[] = ['Kurk', 'Berk', '3D'];
+const DISCARD_WINDOW_MS = 3000;
+
 export default function Stopwatch() {
+  const activitiesQuery = useActivities();
+  const activeSessionsQuery = useActiveSessions();
+  const startSession = useStartSession();
+  const stopSession = useStopSession();
+  const discardSession = useDiscardSession();
+
+  const activities = activitiesQuery.data ?? [];
+
+  // Selection state.
+  const [insoleType, setInsoleType] = useState<InsoleType>('Kurk');
+  const [activeActivityId, setActiveActivityId] = useState<number | null>(null);
+  const [pairCount, setPairCount] = useState(2);
+  const [pairCountText, setPairCountText] = useState('2');
+
+  // Running state — server-authoritative.
+  const [sessionId, setSessionId] = useState<number | null>(null);
+  const [startMs, setStartMs] = useState<number | null>(null);
+  const [isPaused, setIsPaused] = useState(false);
+  // Accumulated paused ms + the moment the current pause began (client-only).
+  const [pausedMs, setPausedMs] = useState(0);
+  const [pauseStartedMs, setPauseStartedMs] = useState<number | null>(null);
+  const [nowMs, setNowMs] = useState(() => Date.now());
+
+  // Double-press discard arming.
+  const [discardPending, setDiscardPending] = useState(false);
+  const discardTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+
+  const isRunning = sessionId !== null;
+
+  // Recover an active session on load (phone-died / resume-elsewhere path).
+  useEffect(() => {
+    if (isRunning) return;
+    const active = activeSessionsQuery.data;
+    if (!active || active.length === 0) return;
+    const session: WorkSession = active[0];
+    setSessionId(session.id);
+    setStartMs(new Date(session.start_time).getTime());
+    setActiveActivityId(session.activity_id);
+    if (session.insole_type) setInsoleType(session.insole_type);
+    setPairCount(session.pair_count);
+    setPairCountText(String(session.pair_count));
+    setIsPaused(false);
+    setPausedMs(0);
+    setPauseStartedMs(null);
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [activeSessionsQuery.data]);
+
+  // Live 1s tick while running and not paused.
+  useEffect(() => {
+    if (!isRunning || isPaused) return;
+    setNowMs(Date.now());
+    const id = setInterval(() => setNowMs(Date.now()), 1000);
+    return () => clearInterval(id);
+  }, [isRunning, isPaused]);
+
+  useEffect(() => {
+    return () => {
+      if (discardTimerRef.current) clearTimeout(discardTimerRef.current);
+    };
+  }, []);
+
+  const filteredActivities = activities.filter((a) => a.insole_types.includes(insoleType));
+  const canStart = activeActivityId !== null;
+
+  // While paused, freeze the clock at the moment the pause began.
+  const effectiveNow = isPaused && pauseStartedMs !== null ? pauseStartedMs : nowMs;
+  const elapsed = startMs !== null ? elapsedSeconds(startMs, effectiveNow, pausedMs) : 0;
+
+  function handleSelectZool(type: InsoleType) {
+    if (isRunning) return;
+    setInsoleType(type);
+    setActiveActivityId(null); // changing the zool clears the chosen handling
+  }
+
+  function handlePairCountText(text: string) {
+    setPairCountText(text);
+    const parsed = parseInt(text, 10);
+    if (!Number.isNaN(parsed) && parsed > 0) setPairCount(parsed);
+  }
+
+  function adjustPairCount(delta: number) {
+    const next = Math.max(1, pairCount + delta);
+    setPairCount(next);
+    setPairCountText(String(next));
+  }
+
+  function handleStart() {
+    if (!canStart || activeActivityId === null) return;
+    startSession.mutate(
+      { activity_id: activeActivityId, insole_type: insoleType, pair_count: pairCount },
+      {
+        onSuccess: (session) => {
+          setSessionId(session.id);
+          setStartMs(new Date(session.start_time).getTime());
+          setIsPaused(false);
+          setPausedMs(0);
+          setPauseStartedMs(null);
+          setNowMs(Date.now());
+        },
+      },
+    );
+  }
+
+  function handleTapDisplay() {
+    if (!isRunning) {
+      if (canStart) handleStart();
+      return;
+    }
+    if (isPaused) {
+      // Resume: fold the just-finished pause span into the accumulator.
+      if (pauseStartedMs !== null) setPausedMs((prev) => prev + (Date.now() - pauseStartedMs));
+      setPauseStartedMs(null);
+      setIsPaused(false);
+    } else {
+      // Pause.
+      setPauseStartedMs(Date.now());
+      setIsPaused(true);
+    }
+  }
+
+  function resetTimer() {
+    setSessionId(null);
+    setStartMs(null);
+    setIsPaused(false);
+    setPausedMs(0);
+    setPauseStartedMs(null);
+    setDiscardPending(false);
+    if (discardTimerRef.current) {
+      clearTimeout(discardTimerRef.current);
+      discardTimerRef.current = null;
+    }
+  }
+
+  function handleStop() {
+    if (sessionId === null) return;
+    const id = sessionId;
+    stopSession.mutate(id, { onSuccess: () => resetTimer() });
+    // Selections (zool/handling/count) persist for the next session.
+  }
+
+  function handleDiscard() {
+    if (sessionId === null) return;
+    if (!discardPending) {
+      setDiscardPending(true);
+      discardTimerRef.current = setTimeout(() => {
+        setDiscardPending(false);
+        discardTimerRef.current = null;
+      }, DISCARD_WINDOW_MS);
+      return;
+    }
+    if (discardTimerRef.current) {
+      clearTimeout(discardTimerRef.current);
+      discardTimerRef.current = null;
+    }
+    const id = sessionId;
+    discardSession.mutate(id, { onSuccess: () => resetTimer() });
+  }
+
+  const statusPill = !isRunning
+    ? canStart
+      ? 'Tik om te starten'
+      : null
+    : isPaused
+      ? 'Gepauzeerd — tik om te hervatten'
+      : 'Tik om te pauzeren';
+
   return (
     <div className="screen">
       <h1 className="screen-title">Stopwatch</h1>
+
+      {/* Section 1 — Type zool */}
+      <h2 className="section-label">Type zool</h2>
+      <div className="segmented" style={{ display: 'flex', gap: 8, marginBottom: 20 }}>
+        {ALL_TYPES.map((type) => {
+          const selected = insoleType === type;
+          return (
+            <button
+              key={type}
+              type="button"
+              disabled={isRunning}
+              aria-pressed={selected}
+              className={selected ? 'seg-btn seg-btn-active' : 'seg-btn'}
+              style={{
+                flex: 1,
+                padding: '12px 4px',
+                borderRadius: 12,
+                fontWeight: 600,
+                cursor: isRunning ? 'not-allowed' : 'pointer',
+                border: `1px solid ${selected ? '#2563EB' : '#E5E7EB'}`,
+                background: selected ? '#EFF6FF' : '#ffffff',
+                color: isRunning ? '#9CA3AF' : selected ? '#2563EB' : '#6B7280',
+              }}
+              onClick={() => handleSelectZool(type)}
+            >
+              {type}
+            </button>
+          );
+        })}
+      </div>
+
+      {/* Section 2 — Type handeling */}
+      <h2 className="section-label">Type handeling</h2>
+      {filteredActivities.length === 0 ? (
+        <p className="muted" style={{ marginBottom: 20 }}>
+          Geen handelingen beschikbaar voor {insoleType} zolen. Voeg ze toe via Instellingen.
+        </p>
+      ) : (
+        <select
+          aria-label="Type handeling"
+          className="field-input"
+          disabled={isRunning}
+          value={activeActivityId ?? ''}
+          onChange={(e) => setActiveActivityId(e.target.value ? Number(e.target.value) : null)}
+          style={{ marginBottom: 20 }}
+        >
+          <option value="">Kies een handeling...</option>
+          {filteredActivities.map((a) => (
+            <option key={a.id} value={a.id}>
+              {a.name}
+            </option>
+          ))}
+        </select>
+      )}
+
+      {/* Section 3 — Aantal zolen */}
+      <h2 className="section-label">Aantal zolen</h2>
+      <div
+        className="stepper"
+        style={{
+          display: 'flex',
+          alignItems: 'center',
+          border: '1px solid #E5E7EB',
+          borderRadius: 12,
+          overflow: 'hidden',
+          marginBottom: 24,
+        }}
+      >
+        <button
+          type="button"
+          aria-label="Minder zolen"
+          disabled={isRunning || pairCount <= 1}
+          onClick={() => adjustPairCount(-1)}
+          style={{ width: 64, padding: 14, border: 'none', background: '#F3F4F6', fontSize: 20 }}
+        >
+          −
+        </button>
+        <input
+          aria-label="Aantal zolen"
+          inputMode="numeric"
+          className="field-input"
+          value={pairCountText}
+          disabled={isRunning}
+          onChange={(e) => handlePairCountText(e.target.value)}
+          style={{ flex: 1, textAlign: 'center', border: 'none', borderRadius: 0 }}
+        />
+        <button
+          type="button"
+          aria-label="Meer zolen"
+          disabled={isRunning}
+          onClick={() => adjustPairCount(1)}
+          style={{ width: 64, padding: 14, border: 'none', background: '#F3F4F6', fontSize: 20 }}
+        >
+          +
+        </button>
+      </div>
+
+      {/* Section 4 — Stopwatch display */}
+      <button
+        type="button"
+        className="stopwatch-display"
+        aria-label="Stopwatch"
+        onClick={handleTapDisplay}
+        style={{
+          display: 'block',
+          width: '100%',
+          textAlign: 'center',
+          padding: '32px 16px',
+          borderRadius: 16,
+          marginBottom: 24,
+          cursor: !isRunning && !canStart ? 'default' : 'pointer',
+          border: `1px solid ${isPaused ? '#FDE68A' : '#E5E7EB'}`,
+          background: '#ffffff',
+        }}
+      >
+        <div
+          style={{
+            fontSize: 64,
+            fontWeight: 700,
+            fontVariantNumeric: 'tabular-nums',
+            color: isPaused ? '#D97706' : isRunning ? '#111827' : '#9CA3AF',
+          }}
+        >
+          {formatTime(elapsed)}
+        </div>
+        {statusPill && (
+          <div
+            style={{
+              marginTop: 8,
+              fontSize: 14,
+              fontWeight: 600,
+              color: isPaused ? '#D97706' : '#2563EB',
+            }}
+          >
+            {statusPill}
+          </div>
+        )}
+      </button>
+
+      {/* Section 5 — Action buttons */}
+      {!isRunning ? (
+        <button
+          type="button"
+          className="btn-primary"
+          disabled={!canStart || startSession.isPending}
+          onClick={handleStart}
+        >
+          Start Stopwatch
+        </button>
+      ) : (
+        <div style={{ display: 'flex', flexDirection: 'column', gap: 12 }}>
+          <button
+            type="button"
+            className="btn-primary"
+            disabled={stopSession.isPending}
+            onClick={handleStop}
+            style={{ background: '#DC2626' }}
+          >
+            Stop & Opslaan
+          </button>
+          <button
+            type="button"
+            onClick={handleDiscard}
+            style={{
+              width: '100%',
+              padding: 16,
+              fontSize: 16,
+              fontWeight: 600,
+              borderRadius: 16,
+              border: 'none',
+              cursor: 'pointer',
+              background: discardPending ? '#374151' : '#F3F4F6',
+              color: discardPending ? '#ffffff' : '#6B7280',
+            }}
+          >
+            {discardPending ? 'Nogmaals tikken ter bevestiging' : 'Annuleren'}
+          </button>
+        </div>
+      )}
     </div>
   );
 }