This chapter focuses on the principal-agent concept as an analytical framework for examining relations between actors within the European Union. This chapter presents the basic premises of this concept, which is based on economic theory and is applied to relations between superiors (principals) and subordinates (agents). This chapter is structured into several parts, which successively present the basic background of the concept, research in this area, the delegation of powers in the EU and its limits, and key aspects such as information asymmetry, control mechanisms, and incentive structures. The main insights of this chapter include the importance of delegation of authority for the effective functioning of institutions, the risks associated with delegation, and the ways in which principals can control the behaviour of agents. This chapter also provides a unified analytical framework that will be used in other parts of the publication to analyse EU security agencies. This framework includes key areas of analysis such as causes of delegation, nature and extent of delegation, agency exercise of authority, information asymmetries, control mechanisms, and incentive structures.