Bolivia and Brazil have universalized their pension and healthcare systems, respectively. Civil society organizations participated actively in social policy expansion, yet they have done so in starkly different ways, reflecting general patterns in each country. Whereas in Brazil, popular participation in social policies takes place through “inside” formal channels, such as conferences and councils, in Bolivia, bottom-up influence occurs mostly via “outside” channels, by coordinating collective action in the streets. Understanding forms of popular participation matters because policies that allow for popular input are potentially more representative, universal, and nondiscretionary. This article argues that differences in the forms of popular participation in social policy expansion can be explained by the characteristics of the institutional context and differences in the types of movements engaged in the policymaking process. By focusing on patterns of participation, these findings add nuance to the literature on Latin America’s welfare states.