In this article, we argue that progress in the study of collective action rests on an increasingly sophisticated application of the social identity approach. We develop the view, however, that the application of this theoretical perspective has been limited by theoretical and empirical difficulties in distinguishing between social categories and psychological groups. These problems have undermined the ability of researchers to correctly specify the collective identities that actually underpin many instances of collective action. As a partial solution to this problem we focus on collective identities based on shared opinion (opinion-based groups). We develop the proposition that much collective action reflects the crystallization or instantiation of opinion-based groups. We also outline an intervention aimed at stimulating commitment to collective action through group-based interaction involving opinion-based group members. We conclude by emphasizing that opinion-based groups tend to be most successful when they present themselves as being representative or aligned with dominant, positively valued social categories such as nations.