This book examines the changing role played by the European Union and international standards on loss and acquisition of nationality. It provides a comparative analysis of EU Member States regulations, administrative practices, court rulings and statistical data on questions related to loss of nationality and European citizenship. It assesses the multifaceted repercussions of the supranational venues of judicial and legal accountability over states autonomy and competences at times of deciding who is and who is not a citizen. The following questions are examined: to what the extent do EU Member States still hold the exclusive competence over domestic decisions in nationality matters? How do international and European legal principles and standards, as well as case-law by European courts progressively affect their margin of manoeuvre at times of deciding who is and who is not a 'citizen'? What are the repercussions of their obligations in safeguarding citizenship of the Union?