Latin America contains roughly half of the world’s countries that use compulsory voting, but this electoral institution has received only limited attention from researchers. This article examines the determinants of voter turnout in the world’s largest electorate subject to mandatory voting, that of Brazil. In analyzing data from six national legislative elections held in Brazil between 1986 and 2006, the study finds that the impact of compulsory laws varies across social and economic groups. From a methodological perspective, the article argues that “compulsoriness” of mandatory voting legislation can be modeled by taking into account both exemptions to the law and the relevance of potential sanctions against non-voters. The issue of enforcement must be considered if we are to develop comprehensive models of electoral participation under conditions of compulsory voting.