This book provides a systematic comparison of the conditions under which sub-state communities in Belgium, Spain and the United Kingdom have demanded and obtained sub-state autonomy. This introductory chapter sets the stage for this by problematising sub-state mobilisation and restructuring in countries undergoing centrifugal sociological decentralisation, i.e. in which autonomy prerogatives are conferred from the state to sub-state entities along sociological lines. It does so by putting this phenomenon into perspective of the so-called ‘paradox of federalism’. After presenting the structure of the book, the chapter summarises the main conceptual and empirical arguments that are made throughout, and articulates them as the building-blocks of what is proposed in this book as a dual constellational theory of centrifugal sociological decentralisation. The theory is constellational in that it explains sub-state mobilisation and restructuring as driven by the interplay of factors from different theoretical strands. It is dual in that it shows sub-state autonomy dynamics to unfold differently in two main types of entities: sub-state communities with strong sociological distinctiveness and historical political grievances (e.g. Flanders, Catalonia or Scotland), and sub-state communities with weaker sociological distinctiveness and historical political grievances (e.g. Wallonia, Asturias or Cornwall).