Time is a crucial dimension in the application of any law. In tax law, however, where an environment characterized by rapid change on the national, European, and international levels complicates the provision of accurate legal advice, timing is particularly sensitive. This book is the first to analyse the relationship between time and three key areas of tax: treaties, EU law, and constitutional law issues, such as legal certainty and individual rights. Among the numerous timing issues arising out of applying tax rules, the book addresses the following: – time limits within which relief must be requested; – statutes of limitation for claiming a tax refund; – transitional issues relating to changes in tax treaties; – attribution of profits and expenses to a moving or closed-down business; – effect of tax-related CJEU decisions and EU directives; – compliance of exit tax regimes with free movement; – limits of retroactivity under principles protected by the EU Charter and the ECHR; and – conflict between efficiency of taxation and individual rights. Derived from a recent conference organized by the prestigious ATOZ Chair for European and International Taxation at the University of Luxembourg, the book brings together contributions from leading tax experts from various areas of tax practice, academia, and the judiciary. Among other issues, the book notably expands on how economic theory can inform a constitutional analysis of the timing of taxation. There is no other work that concentrates so usefully on the difficulties associated with applying tax rules – whether arising from treaties, jurisprudence, or policy – to changing circumstances over time. This book will quickly prove itself to be an indispensable resource for European tax lawyers, policymakers, company counsels, and academics.