Why is turnout higher in some countries and/or in some elections than in others? Why does it increase or decrease over time? To address these questions, I start with the pioneer studies of Powell and Jackman and then review more recent research. This essay seeks to establish which propositions about the causes of variations in turnout are consistently supported by empirical evidence and which ones remain ambiguous. I point out some enigmas and gaps in the field and suggest directions for future research. Most of the research pertains to established democracies, but analyses of nonestablished democracies are also included here.