This dissertation assesses the performance of social policies in decentralized countries. It explores the factors that shape the successful implementation of non-contributory cash transfers and healthcare in Argentina and Brazil, countries in which subnational governments enjoy high levels of authority. The study finds that effective implementation of major national social assistance and services depend in part on partisan alignments across the different territorial levels - subnational governments enhance national policies either when they are political allies of the national government or when the policy has no clear attribution of responsibility and therefore possesses no electoral risk for the opposition. Furthermore, positive policy legacies and strong territorial infrastructure enhance the implementation of national social policies. The empirical foundation for this argument includes a pooled time series analysis of all provinces in Argentina and all states in Brazil and case studies that build on fifteen months of field-research in two provinces and four municipalities in Argentina, and two states and four municipalities in Brazil. In these places, the author conducted 235 original interviews with key national and subnational politicians and almost 150 structured interviews with social policy recipients.