Do international bureaucracies have a meaningful influence on world politics? Using the UN Secretariat and the evolution of UN peacekeeping as an example, this book shows that even international bureaucracies with limited autonomy can shape international politics. Peace operations are the UN's flagship activity. Over the past decades, UN Blue Helmets have been sent all over the globe and have been performing an expanding set of intrusive tasks, while being supported by increasingly professional institutional structures. Silke Weinlich covers these operational, conceptual and institutional dimensions and focuses on three specific decisions that have been crucial to the evolution of UN peacekeeping: the establishment of the UN transitional administration in East Timor, the development of a peacekeeping doctrine, and the establishment of the Standing Police Capacity. With its integrative framework of analysis, this book makes a contribution to the debate between rationalism and constructivism on the agency of international organisations.