Why did the expanded inclusion of previously excluded sectors in Latin America not prevent the eruption of a severe legitimacy crisis? We argue that the reproduction of legitimacy is engendered by political incorporation and inclusionary policies do not engender political incorporation. We propose a concept of political incorporation that we define as the ability of the political system to continuously channel demands and interests of substantive constituencies. Our concept has two necessary dimensions: political rights (a procedural component) and political aggregation (a substantive component). Political aggregation is the effective existence of organized constituencies regarding salient issues and party organizations or leaders with the ability to aggregate and channel demands from these relevant activated constituencies. Our conceptualization allows us to derive a typology of political incorporation and a typology of diminished subtypes. We develop an index of political incorporation and employ it to measure political incorporation in Latin America.