This chapter presents evidence of social identity threat in the underrepresentation of women in STEM. When an identity is perceived to be devalued because of negative stereotypes or a history of bias or exclusion, people can experience social identity threat, the worry and uncertainty that they will be viewed in terms of their social group—and not as an individual. This threat can lead to deleterious consequences for how individuals view themselves, the situation, and their place within it. The authors highlight the environmental cues that trigger or diffuse social identity threat—knowledge that can be used to create more welcoming academic settings for all social groups.