This paper seeks to explain differential participation in social movements. It does so by attempting to bridge structural-level and individual-level explanations. We test a number of hypotheses drawn from the social networks and the rationalist perspectives on individual engagement by means of survey data on members of a major organization of the Swiss solidarity movement. Both perspectives find empirical support: the intensity of participation depends both on the embeddedness in social networks and on the individual perceptions of participation, that is, the evaluation of a number of cognitive parameters related to engagement. In particular, to be recruited by an activist and the perceived effectiveness of one's own potential contribution are the best predictors of differential participation. We specify the role of networks for social movements by looking at the nature and content of networks and by distinguishing between three basic functions of networks: structurally connecting prospective participants to an opportunity to participate, socializing them to a protest is- sue, and shaping their decision to become involved. The latter function implies that the embeddedness in social networks significantly affects the individual perceptions of participation.