diff --git a/app/db/__init__.py b/app/db/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/app/db/models.py b/app/db/models.py
new file mode 100644
index 0000000..f5f8de8
--- /dev/null
+++ b/app/db/models.py
@@ -0,0 +1,92 @@
+from __future__ import annotations
+
+import enum
+from datetime import datetime, timezone
+
+from sqlalchemy import Boolean, DateTime, Enum, ForeignKey, Integer, String, Text, UniqueConstraint
+from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship
+
+
+def utcnow() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+class HealthState(str, enum.Enum):
+    HEALTHY = "healthy"
+    SYNCING = "syncing"
+    DEGRADED = "degraded"
+    ACTION_REQUIRED = "action_required"
+    DISABLED = "disabled"
+
+
+class ActivityStatus(str, enum.Enum):
+    DISCOVERED = "discovered"
+    DOWNLOADED = "downloaded"
+    CONVERTED = "converted"
+    IMPORTED = "imported"
+    DUPLICATE = "duplicate"
+    FAILED = "failed"
+
+
+class SyncRunStatus(str, enum.Enum):
+    RUNNING = "running"
+    SUCCESS = "success"
+    PARTIAL = "partial"
+    FAILED = "failed"
+
+
+class SyncUser(Base):
+    __tablename__ = "sync_users"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    name: Mapped[str] = mapped_column(String(120), nullable=False)
+    enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    health_state: Mapped[HealthState] = mapped_column(Enum(HealthState), nullable=False, default=HealthState.HEALTHY)
+    mywhoosh_state: Mapped[str] = mapped_column(String(32), nullable=False, default="unknown")
+    garmin_state: Mapped[str] = mapped_column(String(32), nullable=False, default="unknown")
+    action_reason: Mapped[str | None] = mapped_column(Text)
+    mywhoosh_email_enc: Mapped[str] = mapped_column(Text, nullable=False)
+    mywhoosh_password_enc: Mapped[str] = mapped_column(Text, nullable=False)
+    garmin_email_enc: Mapped[str] = mapped_column(Text, nullable=False)
+    garmin_password_enc: Mapped[str] = mapped_column(Text, nullable=False)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=utcnow)
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=utcnow, onupdate=utcnow)
+
+
+class Activity(Base):
+    __tablename__ = "activities"
+    __table_args__ = (UniqueConstraint("user_id", "mywhoosh_activity_id", name="uq_activity_user_mywhoosh"),)
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    user_id: Mapped[int] = mapped_column(ForeignKey("sync_users.id", ondelete="CASCADE"), nullable=False, index=True)
+    mywhoosh_activity_id: Mapped[str] = mapped_column(String(255), nullable=False)
+    activity_name: Mapped[str] = mapped_column(String(255), nullable=False)
+    activity_timestamp: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    source_fit_path: Mapped[str | None] = mapped_column(Text)
+    converted_fit_path: Mapped[str | None] = mapped_column(Text)
+    status: Mapped[ActivityStatus] = mapped_column(Enum(ActivityStatus), nullable=False, default=ActivityStatus.DISCOVERED)
+    last_completed_stage: Mapped[ActivityStatus] = mapped_column(Enum(ActivityStatus), nullable=False, default=ActivityStatus.DISCOVERED)
+    retryable: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    garmin_activity_id: Mapped[str | None] = mapped_column(String(255))
+    last_error: Mapped[str | None] = mapped_column(Text)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=utcnow)
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=utcnow, onupdate=utcnow)
+
+
+class SyncRun(Base):
+    __tablename__ = "sync_runs"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    user_id: Mapped[int] = mapped_column(ForeignKey("sync_users.id", ondelete="CASCADE"), nullable=False, index=True)
+    started_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=utcnow)
+    finished_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    status: Mapped[SyncRunStatus] = mapped_column(Enum(SyncRunStatus), nullable=False, default=SyncRunStatus.RUNNING)
+    discovered_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    imported_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    skipped_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    failed_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    summary_error: Mapped[str | None] = mapped_column(Text)
diff --git a/app/db/repositories.py b/app/db/repositories.py
new file mode 100644
index 0000000..b239ff2
--- /dev/null
+++ b/app/db/repositories.py
@@ -0,0 +1,69 @@
+from datetime import datetime
+
+from sqlalchemy import select
+from sqlalchemy.exc import IntegrityError
+from sqlalchemy.orm import Session
+
+from app.db.models import Activity, ActivityStatus, HealthState, SyncUser
+
+
+class UserRepository:
+    def __init__(self, session: Session) -> None:
+        self.session = session
+
+    def create(self, **values) -> SyncUser:
+        user = SyncUser(**values)
+        self.session.add(user)
+        self.session.commit()
+        return user
+
+    def get(self, user_id: int) -> SyncUser | None:
+        return self.session.get(SyncUser, user_id)
+
+    def list_enabled(self) -> list[SyncUser]:
+        return list(self.session.scalars(select(SyncUser).where(SyncUser.enabled.is_(True)).order_by(SyncUser.id)))
+
+
+class ActivityRepository:
+    def __init__(self, session: Session) -> None:
+        self.session = session
+
+    def get_or_create_discovered(
+        self,
+        *,
+        user_id: int,
+        mywhoosh_activity_id: str,
+        activity_name: str,
+        activity_timestamp: datetime | None,
+    ) -> tuple[Activity, bool]:
+        existing = self.session.scalar(
+            select(Activity).where(
+                Activity.user_id == user_id,
+                Activity.mywhoosh_activity_id == mywhoosh_activity_id,
+            )
+        )
+        if existing is not None:
+            return existing, False
+        activity = Activity(
+            user_id=user_id,
+            mywhoosh_activity_id=mywhoosh_activity_id,
+            activity_name=activity_name,
+            activity_timestamp=activity_timestamp,
+            status=ActivityStatus.DISCOVERED,
+            last_completed_stage=ActivityStatus.DISCOVERED,
+        )
+        self.session.add(activity)
+        try:
+            self.session.commit()
+        except IntegrityError:
+            self.session.rollback()
+            existing = self.session.scalar(
+                select(Activity).where(
+                    Activity.user_id == user_id,
+                    Activity.mywhoosh_activity_id == mywhoosh_activity_id,
+                )
+            )
+            if existing is None:
+                raise
+            return existing, False
+        return activity, True
diff --git a/app/db/session.py b/app/db/session.py
new file mode 100644
index 0000000..22f07db
--- /dev/null
+++ b/app/db/session.py
@@ -0,0 +1,18 @@
+from sqlalchemy import create_engine
+from sqlalchemy.engine import Engine
+from sqlalchemy.orm import Session, sessionmaker
+
+from app.db.models import Base
+
+
+def create_db_engine(database_url: str) -> Engine:
+    connect_args = {"check_same_thread": False} if database_url.startswith("sqlite") else {}
+    return create_engine(database_url, connect_args=connect_args, future=True)
+
+
+def create_session_factory(engine: Engine) -> sessionmaker[Session]:
+    return sessionmaker(bind=engine, autoflush=False, expire_on_commit=False)
+
+
+def initialize_schema(engine: Engine) -> None:
+    Base.metadata.create_all(engine)
diff --git a/app/main.py b/app/main.py
index 4931c44..8bebfdf 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1,6 +1,7 @@
 from fastapi import FastAPI
 
 from app.config import Settings, get_settings
+from app.db.session import create_db_engine, create_session_factory, initialize_schema
 
 
 def create_app(settings: Settings | None = None) -> FastAPI:
@@ -12,6 +13,11 @@ def create_app(settings: Settings | None = None) -> FastAPI:
     app = FastAPI(title="MyWhoosh Garmin Sync")
     app.state.settings = resolved
 
+    engine = create_db_engine(resolved.database_url)
+    initialize_schema(engine)
+    app.state.db_engine = engine
+    app.state.session_factory = create_session_factory(engine)
+
     @app.get("/healthz")
     def healthz() -> dict[str, str]:
         return {"status": "ok"}
diff --git a/tests/conftest.py b/tests/conftest.py
index e69de29..c12fe94 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -0,0 +1,30 @@
+import pytest
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session, sessionmaker
+from sqlalchemy.pool import StaticPool
+
+from app.db.models import Base
+from app.db.repositories import ActivityRepository, UserRepository
+
+
+@pytest.fixture
+def db_session() -> Session:
+    engine = create_engine(
+        "sqlite://",
+        connect_args={"check_same_thread": False},
+        poolclass=StaticPool,
+    )
+    Base.metadata.create_all(engine)
+    factory = sessionmaker(bind=engine, expire_on_commit=False)
+    with factory() as session:
+        yield session
+
+
+@pytest.fixture
+def user_repository(db_session: Session) -> UserRepository:
+    return UserRepository(db_session)
+
+
+@pytest.fixture
+def activity_repository(db_session: Session) -> ActivityRepository:
+    return ActivityRepository(db_session)
diff --git a/tests/db/test_repositories.py b/tests/db/test_repositories.py
new file mode 100644
index 0000000..28cdf8d
--- /dev/null
+++ b/tests/db/test_repositories.py
@@ -0,0 +1,54 @@
+from app.db.models import ActivityStatus, HealthState
+
+
+def test_create_two_independent_users(db_session, user_repository) -> None:
+    first = user_repository.create(
+        name="Max",
+        enabled=True,
+        health_state=HealthState.HEALTHY,
+        mywhoosh_email_enc="mw-1",
+        mywhoosh_password_enc="mw-pw-1",
+        garmin_email_enc="g-1",
+        garmin_password_enc="g-pw-1",
+    )
+    second = user_repository.create(
+        name="Anna",
+        enabled=True,
+        health_state=HealthState.HEALTHY,
+        mywhoosh_email_enc="mw-2",
+        mywhoosh_password_enc="mw-pw-2",
+        garmin_email_enc="g-2",
+        garmin_password_enc="g-pw-2",
+    )
+
+    assert first.id != second.id
+    assert {u.name for u in user_repository.list_enabled()} == {"Max", "Anna"}
+
+
+def test_activity_external_id_is_unique_per_user(user_repository, activity_repository) -> None:
+    user = user_repository.create(
+        name="Max",
+        enabled=True,
+        health_state=HealthState.HEALTHY,
+        mywhoosh_email_enc="a",
+        mywhoosh_password_enc="b",
+        garmin_email_enc="c",
+        garmin_password_enc="d",
+    )
+    created, inserted = activity_repository.get_or_create_discovered(
+        user_id=user.id,
+        mywhoosh_activity_id="mw-123",
+        activity_name="Morning Ride",
+        activity_timestamp=None,
+    )
+    same, inserted_again = activity_repository.get_or_create_discovered(
+        user_id=user.id,
+        mywhoosh_activity_id="mw-123",
+        activity_name="Morning Ride",
+        activity_timestamp=None,
+    )
+
+    assert inserted is True
+    assert inserted_again is False
+    assert created.id == same.id
+    assert same.status == ActivityStatus.DISCOVERED