Baltic Journal of European Studies, Vol. 7, iss. 2, (2017), s. 84-106
Initially, before the entry into force of the Maastricht Treaty, differences in integration between members of the European Communities (EC; later the European Union) were relatively few and usually temporary in nature. The Schengen Agreement, the Maastricht Treaty and the Treaty of Amsterdam, and the possibility of establishing enhanced cooperation meant that the problem was becoming more and more important in the functioning of the EU - both in theory and in practice. The objective of the paper is to show that for several years, along with the stagnation in the deepening of integration between all the EU Member States, differentiation of integration in the EU is progressing very rapidly. The progressing differentiation in the EU is a consequence of mainly two processes: the development of enhanced cooperation and reforms in the eurozone, which are strengthened by the widening of the EU. The article covers the issue of the categorization of differentiation of European Union integration, which constitutes the theoretical framework for further considerations. Specified processes which contribute to increasing the differentiation of the EU are discussed, showing the development of enhanced cooperation in the EU and presenting the reforms of the eurozone. The article concludes with the identification and the consequences of differentiated integration, both those that have already occurred and those that may occur in the future.