How do poor people in Latin America participate in politics? What explains the variation in the patterns of voting, protesting, and contacting government for the region’s poorest citizens? Why are participation gaps larger in some countries than in others? This book offers the first large-scale empirical analysis of political participation in Latin America, focusing on patterns of participation among the poorest citizens in each country. Far from being politically inert, under certain conditions the poorest citizens in Latin America act and speak for themselves with an intensity that far exceeds their modest socioeconomic resources. We argue that key institutions of democracy, namely civil society, political parties, and competitive elections, have an enormous impact on whether or not poor people turn out to vote, protest, and contact government officials. When voluntary organizations thrive in poor communities and when political parties focus their mobilization efforts on poor individuals, they respond with high levels of political activism. Poor people’s activism also benefits from strong parties, robust electoral competition, and well-functioning democratic institutions. Where electoral competition is robust and where the power of incumbents is constrained, we see higher levels of participation by poor individuals and more political equality. Precisely because the individual resource constraints that poor people face are daunting obstacles to political activism, our explanation focuses on those features of democratic politics that create opportunities for participation that have the strongest effect on poor people’s political behavior.