This chapter begins the structured comparative analysis with an overview of the pension reforms in Argentina, Brazil, Mexico, and Uruguay. The cases will show the complex and contingent mechanisms of institutional change and highlight how reform outcomes are mediated by the political, economic, and institutional landscape of each country, as well as by the partisan structure of political conflict. Before delving into the question of the causal mechanisms of change, however, we must ask: How much change has actually occurred? As the discussion of the measurement problem in Chapter 2 revealed, this question lacks a straightforward answer. For the output of institutional change – the formal contours of the new pension institutions – differ markedly from the effective reach of the new private pension institutions. Indeed, the latter – measured by active contribution rate within the working-age population – reveals an institutional scope that is considerably narrower than the formal rules of these programs would indicate. Governments also retain a significant financial and regulatory presence in reformed pension markets, and in some cases even compete with private pension fund managers to invest individuals' retirement funds. The ultimate reach of private pension markets in Latin America, moreover, is cast in doubt by the emergence of political challenges to private pension systems in some cases, and by revisions to such programs in countries like Argentina that have curtailed the size of the private pension system. Whether new private pension institutions will endure is unlikely to be known with certainty for some time. Nevertheless, we may gather initial evidence of their solidity from the patterns of compliance with the rules of reformed pension systems.