The principal aim of this book is to revisit the basic theme of “unity and diversity” that remains at the heart of research into federalism and federation. It is time to take another look at its contemporary relevance to ascertain how far the bifocal relationship between unity and diversity has evolved over the years and has been translated into changing conceptual lenses, practical reform proposals and in some cases new institutional practices. This book is structured around four main parts: (1) the evolving conception of diversity over time and across continents; (2) the interplay between unity and diversity in complex settings; (3) federalism as decision-making and new institutional practices that have been put forward and tested; and (4) constitutional design and asymmetrical federalism as a way to respond to legitimate and insisting claims and political demands.