Social movements have become a constitutive part of contemporary societies, especially so in democratic contexts where the institutional conditions allow for movements to be formed and express themselves freely. They involve conflictual relations with clearly identified opponents, are linked by dense informal networks, share a distinct collective identity, and engage primarily – but not exclusively – in protest activities. Explanations of movement mobilization have typically stressed a number of key factors, or combinations thereof: in particular, grievances, mobilizing structures, political opportunities, and framing processes. This chapter provides an overview of some key aspects relating to the study of social movements. Given the breath of this field, it can only be very selective in doing so. The chapter first addresses the question of the definition of social movements. Then it moves on to looking at the ways in which they have been studied. Finally, it briefly discusses what movements leave behind them, that is, the issue of their outcomes and consequences. The chapter concludes with a summary of the most salient aspects addressed and some directions for future research.