This introductory chapter first outlines the aims and history of the international project on Core Concepts in Criminal Law and Criminal Justice. The aims have been inspired by the increasing globalisation of criminal law and criminal justice, which has led to a growing desire to develop common approaches to common problems and to learn from the diversity of current practice in different countries. This has been reinforced by the internationalisation of criminal justice in international and mixed criminal tribunals. There is now a need to engage in a multi-jurisdictional and comparative conceptual analysis not provided by previous comparative projects, which typically focus on specific topics or issues. The chapter then provides an overview of the chapters in the volume, each of which aims to uncover underlying commonalities and differences, and to explore the scope for constructive assimilation or reform. Finally, the chapter comments on plans for the future.