?Europe à la carte?, ?hard core? or ?concentric circles?: There are many variations on differentiated integration as an alternative to the community method of simultaneous and uniform integration. Enshrined in TEU and TEC since the Treaty of Amsterdam, the procedure of enhanced cooperation has not been used until 2010, when 14 Member States broke new ground in the field of divorce law. This offers the opportunity to focus on the legal development of differentiation, taking into account historical as well as political influences. Starting with the early days of European integration, elements of differentiation will be researched in legal developments by using a trifold approach, presenting the historical context, the political development and an assessment regarding differentiation. Already having evidence of differentiation in early stages of integration (Hertenstein, Council of Europe, ECSC and EEC), the Luxembourg Accords of 1966 mark the watershed moment in the legal development: The following period of integration deadlock and the first calls for differentiation have its seeds in this agreement. The Schengen Agreement, being an example of differentiated integration beyond the institutional framework, will be presented as well as the TEU?s opt-out clauses concerning the EMU and social policy. Debates on differentiation and the introduction of enhanced cooperation shaped the development until the Treaty of Nice and will be emphasized on in this master?s thesis. Another focus is on the legal development of the treaties? provisions, on enhanced cooperation and on the current implementation embodied in Articles 20 TEU and 326-334 TFEU. Finally, differentiation will be discussed within the scope of the financial and Euro crises: A comprehensive presentation of the debuts of enhanced cooperation leads to an evaluation of future prospects of this kind of integration. Throughout the thesis, emphasis is also placed on the implications for European citizenship.