Interest organizations have always assumed a prominent place in European politics because they link states with citizens, institutionalize the resolution of social conflicts, regulate important sectors of the economy and society, contribute to workable policy outcomes and socialize members into democratic politics. However, the plethora of interest group studies that exist have not consolidated our understanding of these actors. These analyses have been subject to the vagaries of different theoretical perspectives and vary greatly in their conceptual and methodological frameworks. Consequently, a not always complementary and fragmented series of findings has emerged. This book is a systematic and comprehensive effort at investigating and integrating the state of the art in European interest group research. Combining the analysis of interest group politics in the EU with the comparative study of interest organizations, the book seeks to identify the areas about which there is consolidated knowledge, that are marked by controversies, in which considerable uncertainty exists, that have been omitted from research programs and that ought to be part of future studies. Thereby, it provides a general reflection on how the study of interest group politics can be improved by linking it to the comparative study of governance. This book was published as a special issue of West European Politics.