To study the determinants of centrifugal sociological decentralisation and, doing so, unpack the paradox of federalism, this chapter clarifies the three main conceptual choices made in the book. First, it shows how sub-state mobilisation and restructuring relate to each other and argues that, because they mutually influence each other, a research trying to tap into their determinants needs to study them jointly. Furthermore, it argues that an analysis of sub-state mobilisation requires attention for both regionalist and traditional political parties; and that an analysis of sub-state restructuring needs to assess sub-state autonomy in kind, in degree and by competences. Secondly, the chapter reviews existing theories of sub-state mobilisation and restructuring, and identifies five main theoretical strands: sociological, economic, institutionalist, functionalist and rational-choice. On this basis, a comparative theoretical framework is proposed to analyse in which combination and at which stages of the autonomy demand and conferral process, factors from these theoretical strands appear important. Thirdly and finally, the chapter defines how sub-state communities and their agency are approached in the book, and selects 31 of them in Belgium, Spain and the United Kingdom to be studied between 1801 and 2020.