Comparative constitutional law is a newly energized field in the early 21st century. Never before has the field had such a broad range of interdisciplinary interest, with lawyers, political scientists, sociologists and even economists making contributions to our collective understanding of how constitutions are formed and how they operate. Never before has there been such demand from courts, lawyers and constitution-makers in a wide range of countries for comparative legal analysis. And never before has the field been so institutionalized, with new regional and international associations providing fora for the exchange of ideas and the organization of collaborative projects. This Handbook is one such collaborative project, a small effort to provide an overview of the field. It is inherent that any such effort will be incomplete, and we surely recognize the limitations of any effort to distill such a rich field into a single volume. But we also believe that the time has come for some organization of the various issues and controversies that structure academic and legal debate. As the field matures, such efforts will help to advance scholarship to the next level, by focusing attention on outstanding questions as well as raising awareness of issues worth pursuing in under-analyzed jurisdictions. This Introduction provides a brief history of the field, and wrestles with the definitional issues of the boundaries of the constitution. It then draws out the common themes that emerge from a reading of the chapters, particularly as they relate to patterns of constitutional similarity versus difference, or convergence versus divergence. The conclusion briefly speculates on future directions for the field.