This chapter focuses on the nature and position of agencies within the structure of the European Union. This chapter provides a historical overview of the development of EU agencies, divided into three main periods (waves) and analyses their different types and functions. This chapter examines how agencies contribute to regulation and governance in various areas such as the internal market, security and defence, and justice and home affairs. Furthermore, this chapter deals with the classification of agencies according to their activities and decision-making powers, including regulatory, quasi-regulatory, and information/coordinating agencies. The main findings of this chapter include the importance of agencies for the effective functioning of the EU, their role in the decentralization of administration and regulation, and their contribution to ensuring the single market and security. This chapter also highlights the common features of agencies, such as management boards and executive directors, and their responsibility for transparency and accountability to EU institutions and Member States. This chapter provides a theoretical framework for understanding the dynamics and interactions between agencies and other EU institutions.