The price boom of natural resources in the first decade of the 21st century has been a blessing for all South American economies. For some political regimes, however, it was an institutional curse. In Venezuela, Ecuador, Bolivia, and to a lesser extent Argentina, the commodity boom made possible the emergence of “rentier populism,” a new type of political coalition, based on the economic incorporation of the informal sector and funded by windfall gains from exports of natural resources. Based on a new fiscal structure, rentier populism in turn fostered the intensification of plebiscitarian mechanisms of vertical accountability and neutralized constraints of horizontal accountability. The chapter explores the conditions under which a common exogenous shock, the commodity boom, spread across the region with differential coalitional effects. It also delineates the mechanisms that link the rentier populism coalition to the emergence of plebiscitarian and hegemonic forms of accountability. In order to highlight old and new in rentier populism, the chapter places the coalition in historical perspective, using Guillermo O’Donnell’s earlier work on political economy as a source of contrasts and similarities. Likewise, “plebiscitarian hegemonies” are compared with delegative democracies, the key concept in O’Donnell’s later work on institutions.