diff --git a/src/database/db.ts b/src/database/db.ts
new file mode 100644
index 0000000..1056020
--- /dev/null
+++ b/src/database/db.ts
@@ -0,0 +1,69 @@
+import * as SQLite from 'expo-sqlite'
+import { Measurement, SyncStatus } from '../types'
+
+let db: SQLite.SQLiteDatabase
+
+export async function initDatabase(): Promise<void> {
+  db = await SQLite.openDatabaseAsync('weegschaal.db')
+  await db.execAsync(`
+    CREATE TABLE IF NOT EXISTS measurements (
+      id TEXT PRIMARY KEY,
+      weight REAL NOT NULL,
+      bmi REAL NOT NULL,
+      bodyFatPercentage REAL NOT NULL,
+      fatMass REAL NOT NULL,
+      musclePercentage REAL NOT NULL,
+      muscleMass REAL NOT NULL,
+      visceralFat INTEGER NOT NULL,
+      bmr REAL NOT NULL,
+      measuredAt TEXT NOT NULL,
+      createdAt TEXT NOT NULL,
+      updatedAt TEXT NOT NULL,
+      syncStatus TEXT NOT NULL DEFAULT 'local'
+    );
+  `)
+}
+
+export async function insertMeasurement(m: Measurement): Promise<void> {
+  await db.runAsync(
+    `INSERT INTO measurements (id, weight, bmi, bodyFatPercentage, fatMass, musclePercentage, muscleMass, visceralFat, bmr, measuredAt, createdAt, updatedAt, syncStatus)
+     VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+    [m.id, m.weight, m.bmi, m.bodyFatPercentage, m.fatMass, m.musclePercentage, m.muscleMass, m.visceralFat, m.bmr, m.measuredAt, m.createdAt, m.updatedAt, m.syncStatus]
+  )
+}
+
+export async function getAllMeasurements(): Promise<Measurement[]> {
+  const rows = await db.getAllAsync<any>(
+    'SELECT * FROM measurements ORDER BY measuredAt DESC'
+  )
+  return rows as Measurement[]
+}
+
+export async function getLatestMeasurement(): Promise<Measurement | null> {
+  const rows = await db.getAllAsync<any>(
+    'SELECT * FROM measurements ORDER BY measuredAt DESC LIMIT 1'
+  )
+  return rows.length > 0 ? (rows[0] as Measurement) : null
+}
+
+export async function updateMeasurement(id: string, syncStatus: SyncStatus): Promise<void> {
+  await db.runAsync(
+    'UPDATE measurements SET syncStatus = ?, updatedAt = ? WHERE id = ?',
+    [syncStatus, new Date().toISOString(), id]
+  )
+}
+
+export async function upsertMeasurement(m: Measurement): Promise<void> {
+  await db.runAsync(
+    `INSERT OR REPLACE INTO measurements (id, weight, bmi, bodyFatPercentage, fatMass, musclePercentage, muscleMass, visceralFat, bmr, measuredAt, createdAt, updatedAt, syncStatus)
+     VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+    [m.id, m.weight, m.bmi, m.bodyFatPercentage, m.fatMass, m.musclePercentage, m.muscleMass, m.visceralFat, m.bmr, m.measuredAt, m.createdAt, m.updatedAt, m.syncStatus]
+  )
+}
+
+export async function getUnsyncedMeasurements(): Promise<Measurement[]> {
+  const rows = await db.getAllAsync<any>(
+    "SELECT * FROM measurements WHERE syncStatus = 'local' ORDER BY updatedAt ASC"
+  )
+  return rows as Measurement[]
+}
diff --git a/tests/database.test.ts b/tests/database.test.ts
new file mode 100644
index 0000000..0a78538
--- /dev/null
+++ b/tests/database.test.ts
@@ -0,0 +1,79 @@
+import { initDatabase, insertMeasurement, getAllMeasurements, getLatestMeasurement, updateMeasurement } from '../src/database/db'
+import { Measurement } from '../src/types'
+import * as SQLite from 'expo-sqlite'
+
+jest.mock('expo-sqlite', () => ({
+  openDatabaseAsync: jest.fn(),
+}))
+
+const mockMeasurement: Measurement = {
+  id: '550e8400-e29b-41d4-a716-446655440000',
+  weight: 75.5,
+  bmi: 22.3,
+  bodyFatPercentage: 15.2,
+  fatMass: 11.5,
+  musclePercentage: 40.1,
+  muscleMass: 30.3,
+  visceralFat: 8,
+  bmr: 1650,
+  measuredAt: '2024-01-01T08:00:00.000Z',
+  createdAt: '2024-01-01T08:00:00.000Z',
+  updatedAt: '2024-01-01T08:00:00.000Z',
+  syncStatus: 'local',
+}
+
+describe('database', () => {
+  let db: any
+
+  beforeEach(() => {
+    db = {
+      execAsync: jest.fn(),
+      runAsync: jest.fn(),
+      getAllAsync: jest.fn(),
+    }
+    ;(SQLite.openDatabaseAsync as jest.Mock).mockResolvedValue(db)
+  })
+
+  it('initDatabase creates the measurements table', async () => {
+    await initDatabase()
+    expect(db.execAsync).toHaveBeenCalledWith(
+      expect.stringContaining('CREATE TABLE IF NOT EXISTS measurements')
+    )
+  })
+
+  it('insertMeasurement inserts a row', async () => {
+    db.runAsync.mockResolvedValue(undefined)
+    await initDatabase()
+    await insertMeasurement(mockMeasurement)
+    expect(db.runAsync).toHaveBeenCalledWith(
+      expect.stringContaining('INSERT INTO measurements'),
+      expect.arrayContaining([mockMeasurement.id])
+    )
+  })
+
+  it('getAllMeasurements returns all measurements ordered by measuredAt desc', async () => {
+    db.getAllAsync.mockResolvedValue([{ ...mockMeasurement }])
+    await initDatabase()
+    const result = await getAllMeasurements()
+    expect(result).toHaveLength(1)
+    expect(result[0].id).toBe(mockMeasurement.id)
+  })
+
+  it('getLatestMeasurement returns the most recent', async () => {
+    db.getAllAsync.mockResolvedValue([{ ...mockMeasurement }])
+    await initDatabase()
+    const result = await getLatestMeasurement()
+    expect(result).not.toBeNull()
+    expect(result!.id).toBe(mockMeasurement.id)
+  })
+
+  it('updateMeasurement updates syncStatus', async () => {
+    db.runAsync.mockResolvedValue(undefined)
+    await initDatabase()
+    await updateMeasurement(mockMeasurement.id, 'synced')
+    expect(db.runAsync).toHaveBeenCalledWith(
+      expect.stringContaining('UPDATE measurements'),
+      expect.arrayContaining(['synced', mockMeasurement.id])
+    )
+  })
+})