This introduction presents an overview of some key concepts discussed in the subsequent chapters of the book. This book addresses a variety of crucial constitutional challenges as well as modes of change, analysed as dimensions of a more general tendency towards 'constitutional acceleration', that is, the increased propensity of different actors to engage in (formal) reform of the constitutional order. Constitutional reform is high on the political agenda in European Union member states and Europe at large, as well as beyond the European context, in other parts of the world. The book discusses singular and highly significant case studies in 'established' democracies in Western Europe, exploring and analyzing recent processes of constitutional reform. It describes distinctive, problematic case studies in 'new' democracies in East-Central Europe, analyzing constitutional reform and revision in the context of recently established democratic-constitutional regimes.