This chapter focuses on how individuals’ social identities and the environment interact to influence their experiences in science, technology, engineering and maths (STEM) contexts and, in turn, their decisions to enter and stay in STEM. It introduces new questions that arise from applying a person-by-situation, social-contextual approach to the problem of women’s underrepresentation in STEM. The chapter argues that how one’s social identity is perceived and valued in STEM classrooms and careers plays a powerful role in whether women seek and remain in the environments. Social identity concerns have a deleterious impact on important psychological and behavioural outcomes, which have been especially well documented for women in STEM. Social identity threat research clearly demonstrates that situational cues have a significant impact on the appraisals of educational environments and people’s experiences within them. However, some settings may not be perceived as identity-threatening because they offer few situational cues that point to the devaluation of important social identities.