Four aspects of mobilization are distinguished: formation of mobilization potentials, formation and activation of recruitment networks, arousal of motivation to participate, and removal of barriers to participation. Four steps toward participation in social movements are then distinguished: becoming part of the mobilization potential, becoming target of mobilization attempts, becoming motivated to participate, and overcoming barriers to participation. The relevance of these distinctions is justified theoretically by the claim that different theories are needed to explain separate aspects of mobilization and participation, and practically with the argument that different efforts are required from movement organizations depending on which aspect they are handling. Empirical support from research on mobilization and participation in the Dutch peace movement is presented. Nonparticipation in a mass demonstration can be based on four grounds: lack of sympathy for the movement, not being the target of a mobilization attempt, not being motivated, and the presence of barriers. These results are interpreted in terms of the literature on mobilization and participation.