This book applies the analytical approach called Historical Institutionalism (HI)- so far mostly used within comparative politics-to the field of International Relations (IR). It provides an introduction to HI concepts and makes an argument for why it is particularly well-suited for understanding current developments within international institutions. In particular, it helps us to understand the combination of change and stability that together form the dynamics of institutional development over time. It is the first book to collect original, empirical research applying historical institutionalism to international institutions. The chapters cover a range of institutions important to IR, including the development of European Union competition policy, the global politics of financial reform after the 2008 crisis, the institutional development of the World Health Organization, membership reforms in the League of Nations and the United Nations Security Council, and civil society access to intergovernmental organizations. The concluding chapter discusses the relationship of HI to other institutionalist approaches and the role of HI in future IR research.