Since the early 1990s, European integration has become increasingly differentiated. Analysing the conditions under which member states make use of the opportunity to opt out of, or exclude other countries from, European integration, we argue that different explanations apply to treaty and accession negotiations, respectively. Threatening to block deeper integration, member states with strong national identities secure differentiations in treaty reform. In enlargement, in turn, old member states fear economic disadvantages and low administrative capacity and therefore impose differentiation on poor newcomers. Opt-outs from treaty revisions are limited to the area of core state powers, whereas they also occur in the market in the context of enlargement.