This thesis focuses on women's group organising (WG) in British unions. WGs are broadly defined as collective organising by women that responds to their concerns and need for access to empowering (social) positions. As a 'radical' form, WGs contrast with the formal liberal democratic principles underpinning much union organisation. The need for their examination is stressed by the continued feminisation of the workplace and many union memberships; growing realisation of the need for unions to connect their revitalisation to being responsive to women; and women's on-going experience of inequities in various settings. While existing works provide insights into why women collectivise in union and other contexts, a review of the related literature in industrial relations, women's studies, political studies, sociology and social psychology revealed the absence of an integrated body of work on union WGs. Consequently, the main objective of this study is to provide an empirical and conceptual contribution by addressing the following major questions: • What union factors influence the number and overall 'shape' ofWGs?; • What aims do WGs pursue, how do they address them and what equality ideas inform them?; and • What impacts do WGs make on gender equality in their union? A study of two major British unions, MSF and USDA W, examining seven of their WGs was undertaken from a constructivist-feminist standpoint. Analyses of interview, observational and documentary evidence were guided by two frameworks: i) the dimensions of Hyman's (1994) model of union organisation which were extended by this study's iterative data collection-analysis process, and ii) an independently-derived typology of gender equality ideas which could inform WGs' pursuit of their substantive aims. Criteria for assessing WGs' impact on women's situation in the union setting were developed from existing literature and the data sources. The findings illuminated hitherto unchartered aspects of union operations, and specifically, how WGs influence, and are influenced, by them. Four main conclusions emerge from the findings. First, particular features of the union setting have a key, if not often exclusive, influence on WG arrangements. Second, different WG types emphasise different aims but there is also some overlap in their aims and the equality ideas which inform them. This stresses the complex character and relations between the studied phenomena, and their location within a wider women's structure. Third, WGs pursue a wider range of aims, via uncoordinated equality approaches, than is formally recognised. Their impacts are more extensive than is officially reported, relating to union structure and democracy, agendas, interest representation, power, and social processes and modes of operating. This emphasises how the under-exposure of women's activism can act to under-estimate their efforts and effects for women in the union. Furthermore, each WG aim is usually underpinned by a mixture of unarticulated and dynamic conceptions of gender equality though a slow shift by WGs toward more ambitious ideas about equality is identified. Fourth, while WGs pursue and achieve more than was previously realised for union women, their current operations still seem unlikely to achieve the fundamental union transformation that is needed to achieve 'long' equality (see Cockburn 1989). Equal power sharing by male and female unionists will require the centring of WGs in union strategies that question the basis of union organisation. WGs also need to pursue considered, if multidimensional, approaches to gender equality. This may necessitate that WGs and unions undertake more innovative measures than is currently the case (e.g. more extensive links with community and social movements, WG organisation outside the union).