This part is intended as an introduction to the monograph that explores the evolution and impact of European Union (EU) agencies in the security domain. The introduction briefly presents the topic and its relevance in research and presents the aims of the monograph, outlining the structure and rationale behind the selected approach. The research topic is put into the context of existing scientific literature and the ongoing debate on the EU agencies. The book itself is structured into three parts: the first part provides a theoretical background on EU institutions and the principal-agent model; the second part offers a comparative analysis of 13 EU security agencies; and the final part discusses the findings and future role of these agencies. The analysis focuses on the establishment, institutional structure, execution of powers, and responses to security challenges, highlighting the agencies’ indispensable role in enhancing EU security.