Gender relations and gendered power relations are major defining features of science and technology. This article addresses the question of how to understand gender, and considers their various implications for science and technology. Gender and gender relations can be understood as operating and as relevant to science and technology at several levels: who does science and technology; how science and technology are organized; and the con- struction of knowledge in science and technology. We review five underlying formulations that inform both policy interventions and theorizing around gender and science — gender based on sex; masculinity/femininity and sex roles; categoricalism, structure and plural structures; poststructuralist, discursive and deconstructive approaches; the material-discursive.