diff --git a/backend/app/models/base.py b/backend/app/models/base.py
new file mode 100644
index 0000000..b5f3d84
--- /dev/null
+++ b/backend/app/models/base.py
@@ -0,0 +1,19 @@
+"""Base model and common mixins."""
+from datetime import datetime
+from sqlalchemy import Column, Integer, DateTime, String
+from app.database import Base
+
+
+class TimestampMixin:
+    """Adds created_at and updated_at columns."""
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+
+class TenantMixin:
+    """Adds tenant_id column for multi-tenant isolation.
+
+    Every domain model inherits this to guarantee row-level tenant isolation.
+    Queries must always filter by tenant_id.
+    """
+    tenant_id = Column(Integer, nullable=False, index=True)