The institutional development of European Union (EU) agencies is striking. Over the past decades, forty-six EU agencies have been established to support the European Commission and member states in their regulatory and executive tasks. Today, EU agencies are a vital part of the EU’s administrative capacity. EU agencies have received considerable scholarly attention that used a myriad of theoretical approaches—ranging from institutional, organizational, and bureaucratic reputation to interest group theories—to explain why EU agencies have been created; how they develop over time; whether they are wielders of supranational or intergovernmental power; how they legitimize themselves and cultivate a positive bureaucratic reputation; and how they form alliances or insulate themselves from specific stakeholders. This chapter reviews the rise of EU agencies and introduces a selection of theoretical perspectives that have been used by EU agency scholars to study EU-level agencification and EU agency behaviour, regulatory processes, and outputs.