Abstract: The main objective of this study is an explanation of the main theoretical principles and models of flexible/differentiated integration and their place in the historical development of the EU on the basis of political science. The study is mainly a historical and politological analysis and synthesis of the development of Europe’s integration viewed from the aspect of differentiated and flexible integration. On this basis we want to define the so-called wide and narrow conceptions of differentiated and flexible integration, including its partial models, while paying special attention to the theoretical model of the ‘opt-out’ decision.

Keywords: EU, national Identity, the ‘opt-out’ decision