The first chapter deals with the research of institutions in the context of theories of European integration. The aim of this chapter is to provide readers with an overview of different theoretical approaches to European integration and their application to research on EU institutions. This chapter is structured into several parts that successively present the main theories of integration: federalism, functionalism, neo-functionalism, and new institutionalism. Each part focuses on the key concepts and arguments of individual theories and their importance for understanding the functioning of the EU institutions. The main findings of this chapter include the diversity of theoretical approaches to European integration, their historical development and their application to the analysis of EU institutions. This chapter highlights the importance of a theoretical framework for understanding the complex relationships and dynamics within European institutions.