This paper discusses the incidence of gender mainstreaming, and the extent to which it is proving an effective means of facilitating the incorporation of the needs of women, and men, in the operation of the spatial planning system in the United Kingdom. The paper is illustrated by a study undertaken for the Royal Town Planning Institute (RTPI) on the scope and nature of gender mainstreaming within local spatial planning departments. It also draws on a review of national planning guidance documents and other research in which the author is involved, on gender, ethnicity, diversity and disability. Key concepts are defined and misgivings discussed. The emphasis upon women's needs within the concept of gender, and the need to prioritise gender within the equalities and diversity agendas, are explained. The gendered nature of the built environment is outlined. The problem has long been identified by adherents of the women and planning movement. But policy change has proved elusive. Gender mainstreaming has been welcomed as a means of achieving change. But the process has proved conceptually problematic, of questionable efficacy and difficult to implement. The research incorporated an investigation of the extent to which gender is being mainstreamed into planning policy and practice. The effects of European Union directives and United Kingdom government equality initiatives upon national, regional and local planning level mainstreaming activities were investigated. A range of local planning authorities was chosen to provide case studies that showed the different approaches adopted. A qualitative approach was used to tease out those factors that inhibit or enable the mainstreaming process. Although equalities mainstreaming programmes are widespread within local authorities, the majority are generic rather than specifically gendered in emphasis. They are primarily concerned with personnel and procedural matters rather than spatial policy issues. The majority of planning authorities manifest limited understanding of either gender considerations or the process of mainstreaming. Nevertheless, a significant minority of authorities was found where progress is underway. However, progress is hampered by lack of central government policy direction and statutory support. A ‘Gender Mainstreaming Toolkit’ was developed to facilitate the process. It is concluded that this is likely to prove ineffective unless fundamental changes take place within the planning system. In order to achieve effective gender mainstreaming, the following prerequisites are identified: conceptual clarity, stronger policy guidance and legal support at central government level, educational change, and organisational and procedural changes within local planning authorities.