The threat of being negatively stereotyped in mathematics can impair the performance of women on difficult math tests, a phenomenon referred to as stereotype threat (ST). This phenomenon may help to explain why there are fewer women than men pursuing and succeeding in scientific careers. Although ST effects have emerged repeatedly in studies with college-aged women, far less research to date has examined when and how ST affects the performance of girls in this domain. In this paper we present evidence that gender stereotypes, particularly implicit stereotypes, begin to form in early elementary school and provide a continuing, yet in many ways ``hidden'' barrier to girls' future advancement in scientific domains. We then review both lab and field studies showing that children as young as 5 years of age are susceptible to the influence of both negative and positive stereotypes. The implications of these findings for tackling the thorny issue of ST and for educational interventions are discussed.