Throughout the twentieth century, much of the population in Latin America lacked access to social protection. Since the 1990s, however, social policy for millions of outsiders - rural, informal, and unemployed workers and dependents - has been expanded dramatically. Social Policy Expansion in Latin America shows that the critical factors driving expansion are electoral competition for the vote of outsiders and social mobilization for policy change. The balance of partisan power and the involvement of social movements in policy design explain cross-national variation in policy models, in terms of benefit levels, coverage, and civil society participation in implementation. The book draws on in-depth case studies of policy making in Argentina, Brazil, Chile, and Mexico over several administrations and across three policy areas: health care, pensions, and income support. Secondary case studies illustrate how the theory applies to other developing countries.