This article seeks to explain differential participation in social movements. People are not brought to collective action at the same level of intensity. Some become core activists, while others invest only little time and effort. We test a number of hypotheses drawn from the social networks and the rationalist perspectives on individual engagement by means of survey data on participants in a major organization of the Swiss solidarity movement. Not surprisingly, both perspectives find empirical support: the intensity of participation depends both on the embeddedness in social networks and on the individual preferences towards engagement, that is, the perception of a number of aspects implied by engagement. In particular, to be recruited by an activist and the perceived effectiveness of one's own prospected 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: recruitment, socialization, and the bridging of structural and individual components of participation. The latter function suggests that individual preferences are not given a priori but are strongly affected by the m edd dness in social networks. This allows us to criticize both social networks and rational choice accounts of individual participation. Explanations that stress the role of networks are often mechanistic insofar as they fail to show how the mbeddedeness of individuals in pre-existing networks affect their interests, preferences, and utilities. Rationalist accounts, on the other hand, neglect the origin of those interests, preferences, and utilities, which are strongly affected by social relations.