Hugh Clegg’s Trade Unionism under Collective Bargaining was published nearly forty years ago. It is far from being just a work of antiquarian interest, however. Its core argument, that the main influence on trade-union behaviour is the structure of collective bargaining, which depends on the role of employers and their organizations, remains as challenging as it ever was. Its approach, comparative and historical, is a watershed in the theoretical development of industrial relations, paving the way for an emphasis on theory ‘in’ rather than theory ‘of’. It also implicitly raises two questions of enduring significance. The first is the wider contribution of collective bargaining and what its decline means not just for trade-union members but also society as a whole. The other is the conditions necessary for the survival of collective bargaining. The policy implication is that, if society wants to have the benefits of collective bargaining, there will be a need for legislation to boost collective bargaining’s ‘legitimacy power’ to make up for the decline in its ‘coercive power’.