European integration continues to deepen despite major crises and attempts to take back sovereignty. A growing number of member states are reacting to a more constraining EU by negotiating opt-outs. This book provides the first in-depth account of how opt-outs work in practice. It examines the most controversial cases of differentiated integration: the British and Danish opt-outs from Economic and Monetary Union and European policies on borders, asylum, migration, internal security and justice. Drawing on over one hundred interviews with national representatives and EU officials, the author demonstrates how representatives manage the stigma of opting out, allowing them to influence even politically sensitive areas covered by their opt-outs. Developing a practice approach to European integration, the book shows how everyday negotiations transform national interests into European ideals. It is usually assumed that states opt out to preserve sovereignty, but Adler-Nissen argues that national opt-outs may actually reinforce the integration process.