This chapter reviews emerging research that investigates how sub-national government tiers are partitioned geographically, and how this affects political and economic outcomes, with a particular focus on the developing world. Many developing countries, particularly in sub-Saharan Africa, have experienced substantial changes in the number and shape of provincial, regional, or local government units. This phenomenon of ``administrative unit proliferation'' has dramatically reorganized the territorial structure of government and reshaped political processes and outcomes, yet is conceptually distinct from vertical decentralization reforms. The literature has identified functional, political supply-side, and political demand-side arguments for understanding changes in the number of governing units. Functional arguments rely on the logic of efficiency in public goods provision to determine the optimal size of governing units. Supply-side argument focus in the political benefits of administrative unit proliferation for national executives as a tool for patronage, divide-and-rule strategies or as credible commitment device. Demand side arguments highlight the role of marginalized ethnic groups in demanding new governing units to improve representation and accountability. Research on the effects of unit proliferation has been limited and focused on the trade-offs between governing units’ size and the quality of services provision, effects on political violence, and political accountability.