The chapters of this book are grouped loosely into three sections. In the first part, the emphasis is on the variety of interpretations of the two concepts (chapters 2 – 5). The second part deals with new theoretical approaches and research questions (chapters 6 – 10). And the third part addresses methodological problems and innovations with respect to the study of boundary-crossing activities and affiliations (chapters 11 – 14). The book is framed by an introductory essay that connects the strands of the debate (chapter 1) and concluding reflections on how empirical research perspectives may enhance our understanding of the evolution of transnational membership norms in democratic polities (chapter 15).