Recent years have seen British trade unions regain some of the membership and influence lost during the 1980s and 1990s. During the same period, sexual orientation activism and other 'new' social movements have become increasingly prominent on the world stage. This thesis is an empirical examination of British unions' responses to the issues, concerns and priorities of lesbian and gay workers seeking to further their agendas through collective action. In response to the paucity of academic research in this area, this thesis presents an analysis of six unions' attempts to organise and represent diverse lesbian and gay interests within class-focused bureaucratic structures. Establishing a context for this study, the thesis considers historical and contemporary accounts of union organisation, the fusion of class with status-based identities and examples of gender, 'race' and disability action within British trade unions. Through interviews with key actors, the research uncovers the existence of separate 'safe spaces' for lesbian and gay organisation, structures connecting action to 'mainstream' activities, and top-down initiatives designed to promote acceptance of organisational diversity. This thesis examines anticipated 'pockets of resistance' within the unions to sexuality action, but finds the bureaucratic and hierarchic nature of union organisation a larger barrier to the effectiveness of action than any internal opposition. The need to recognise intra-group diversity and the dangers of centralised 'servicing' to the unions' future plans are discussed, in light of contemporary trends towards workplace organisation, partnership and the requirement to address members' heterogeneous needs, as are the questions such an analysis raises for conceptualisations of democratic practice. Through the examination of the unions' organisation of sexual minorities, this thesis refines and extends Kelly's (1998) mobilisation theory and presents an alternative framework of trade union action as 'mobilisation within mobilisation'. It closes by considering the implications for both social movements and continued union 'renewal' of organised labour's attempts to negotiate 'the bigger picture'.