Over the last thirty years, international political economy and international relations have become increasingly sophisticated, both empirically and theoretically. Realist, liberal, and constructivist theorists have developed research programs that yield new insights into some of the most perplexing areas of international politics: the interplay between conflict and cooperation, the impact of domestic political structures on foreign policy, the role of institutions, and the influence of worldviews and causal beliefs on decision-making. In exploring these developments, this book also considers them from the perspectives of security studies, organization theory, and economics. This is a republication in book form of a special fiftieth anniversary issue of the journal International Organization.