Differentiation has become a salient feature of European integration. Yet systematic empirical evidence is lacking about its origins, duration and variation across countries and policies. This article provides such evidence from a new data set on differentiation in European Union treaty law. In addition, it is argued that two logics of treaty‐based differentiation are at work. ‘Instrumental differentiation’ originates in enlargement and is motivated by efficiency and distributional concerns. ‘Constitutional differentiation’ has its origins in treaty revisions and is motivated by concerns about national sovereignty and identity. It is driven by Eurosceptic Member States that are opposed ideologically, or fear popular resistance, to the supranational centralization of core state powers.