This chapter introduces the book's main theoretical argument: that armed groups are dependent on popular support and accordingly strive to obtain and maintain it, efforts which shape insurgents’ repertoire of contention. It outlines the concept of the 'constituency' (Malthaner 2011) which serves as a relational framework to understand the dynamic relations between insurgent groups and their supporters. It further develops the concept by more explicitly developing its spatial dimensions. It argues for a more nuanced understanding of insurgent movements use of space and how it shapes interactions with their supporters. It embeds this theoretical approach in the broader literatures concerned with insurgent groups’ relations with civilians, ranging from counterinsurgency and social movements to rebel governance. It argues that the paradigm of territorial control (Kalyvas, 2006) is too reductive and cannot account for patterns of support for insurgents in areas they do not control. It also addresses the critical role of the state in shaping insurgent behaviour and how state–insurgent interactions are reciprocally formative. It proceeds to look at issues of insurgent governance, recruitment and civilian agency.