This study shows the centrality of women’s organizing to unionism and women’s experience of unions, and provides insights into the circumstances necessary for women’s sustained activism. It examines union operations and how women’s groups influence, and are influenced by, them. It synthesizes research and theory from different literatures, including industrial relations, gender studies and social psychology. It contributes an original analysis of the organizational ‘identity’ of individual unions and women’s groups. It also examines the complex relations between unions and their women’s groups within particular institutions, including the little-examined area of women’s engagement in less formal as well as mainstream union activity. Finally, it develops a number of key recommendations for both women’s groups and union strategy, firmly based on the empirical findings.