This book provides a guide to the major thinkers, concepts, approaches, and debates that have shaped contemporary international legal theory. The book explores key questions and debates in international legal theory, offers new intellectual histories for the discipline, and provides fresh interpretations of significant historical figures, texts, and theoretical approaches. It considers many issues from the field of international legal theory, and provides a guide to the main themes and debates that have driven theoretical work in international law. The text features an introductory chapter (Theorizing International Law) followed by forty-eight chapters which aim to reflect the richness and diversity of this dynamic field. The book is divided into four parts organized around four themes: histories (Part I), approaches (Part II), doctrines and regimes (Part III), and debates (Part IV). The chapters in Part I, introduce some of the key theories and thinkers that are perceived to have provided the foundations of international legal theory and aim to create a methodological awareness of the historical dimension of that theory. The chapters in Part II reflect some of the different ways of categorizing approaches to the theory field. The chapters in Part III provide an overview of theoretical discussions relating to core doctrines and areas of contemporary international law whilst those in Part IV present some of the most existential and essential questions informing the discipline’s current state and likely future.