This text offers a comprehensive overview of the content and development of major theoretical approaches to European integration, as well as critical assessment of current theoretical debates, and an assessment of their value in conducting empirical research. Chapters offer insights into the development, achievements and problems of the respective approaches and follow a set pattern which allows for easy comparision between approaches based on 'bestcases' and 'testcases'. Written by major scholars, it will provide a state-of-the-art handbook of integration theory.