This chapter provides the theoretical underpinnings for an analysis of the impact made by populist radical right parties in local government. This theoretical framework is grounded not just in theories particular to the party family, but also ones more general to the analysis of party politics and institutions. Four main questions are addressed. First, regarding the policy goals of the populist radical right: what do they want? Second, regarding partisan influence over government policy, do parties matter – even in local government? Third, regarding the mainstreaming of the populist radical right, are they tamed by power? And fourth, regarding the strategic uses of local government made by parties, are such localities laboratories of radicalism? Before the subsequent chapters analyse the different routes taken by populist radical right-led local governments in Western Europe, this chapter provides several explicit expectations for how their impact is likely to vary cross-nationally according to two main factors: institutional constraints and central party strategies.