As a distinct type of international organization, regional organizations with their geographical membership criteria have a considerable impact on the lives of many people throughout all regions around the world. Therefore, regional organizations (ROs) have fascinated us for quite some time. Our collaboration started out a few years back with the idea of writing a short paper that happened to turn into a major data collection exercise that kept us busy for several years and culminated in a broad array of different research questions and interests.

In this book, we have pursued a basic interest. Despite the high number of ROs that exist today, there are very few comparative studies. Hence, we wanted to contribute to the vibrant debate by mapping how ROs have developed over time in a systematic and comprehensive manner that still allows for in-depth insights. Thereby we uncovered intriguing global trajectories and regional particularities that we present here.

We are grateful to have received funding for our endeavours. This book brings together the work of two research projects dealing with regional organizations. The first project, Towards an Increasing Regionalization of International Politics? Comparing the Development of External and Internal Competencies of Regional Organizations over Time, was supported by the Fritz Thyssen Foundation (Az.10.16.1.012IB). It provided the resources necessary to collect the raw data for this book. The second project, Overlapping Regionalism in Africa, the Americas, Asia and Europe Compared, received funding from the German Research Council (DFG, grant number PA 1257/7-1; 421167407). This project allowed us to further work with, and build on, the database and to examine the implications arising from the complexity that characterizes contemporary regionalism around the globe.

In addition, we would like to thank our colleagues who provided comments to initial project ideas as well as papers during various conferences and academic meetings. We are indebted to them for their various contributions; in particular, Tanja A. Börzel, Cassandra V. Emmons, Julia Gray, Yoram Haftel, Stephanie C. Hofmann, Tobias Hofmann, Anja Jetschke, Arie M. Kacowicz, Tobias Lenz, Mathis Lohaus, Johannes Muntschick, Nina Reiners, Frederik Söderbaum, Edward Stoddard and Patrick Theiner. Three reviewers from Bristol University Press (BUP) offered thoughtful and detailed comments. We would like to thank them for the opportunity to make this book more accessible and, hopefully, even more convincing.