The 21st century promises to be an "Age of Mobility". More people around the globe, from an ever greater variety of backgrounds, are migrating. As Europe and North America absorb larger and more diverse inflows, many policymakers, commentators, and academics are questioning whether their societies can cope with the influx. Citizenship has emerged as one of the key policy battlegrounds for such concerns. Citizenship lies at the nexus of a host of social policy issues because it provides definitions of identity, belonging, and participation in key aspects of society, including the right to vote. Governments recognize the urgent need to understand citizenship better. This book is the first major work of the Transatlantic Council on Migration, a group of leading political figures, policymakers, and innovative thinkers from the United States and Europe. The contributors provide insights into key aspects of the citizenship debate from a policy perspective. Contributors include: Hans Martens (chief executive, European Policy Centre), John Mollenkopf (director, Center for Urban Research at the Graduate Center, City University of New York), and Demetrios G. Papademetriou (president, Migration Policy Institute). This title is presented in collaboration with the European Policy Centre and the Migration Policy Institute.