This volume is intended to help readers understand the relationship between international law and international relations (IL/IR). As a testament to this dynamic area of inquiry, new research on IL/IR is now being published in a growing list of traditional law reviews and disciplinary journals. The excerpted articles in this volume, all of which were first published in International Organization, represent some of the most important research since serious social science scholarship began in this area more than twenty years ago. They are important milestones toward making IL/IR a central concern of scholarly research in international affairs. The contributions cover some of the main topics of international affairs to provide readers with a range of theoretical perspectives, concepts, and heuristics that can be used to analyze the relationship between international law and international relations.