Researchers from economics, sociology, psychology, and other disciplines have studied the persistent underrepresentation of women in science, technology, engineering, and mathematics (STEM). This chapter summarizes this research. It argues that women’s underrepresentation is concentrated in the math-intensive science fields of geosciences, engineering, economics, math/computer science, and physical science. Its analysis concentrates on the environmental factors that influence ability, preferences, and the rewards for those choices. The chapter examines how gendered stereotypes, culture, role models, competition, risk aversion, and interests contribute to the gender STEM gap, starting in childhood, solidifying by middle school, and affecting women and men as they progress through school and higher education and into the labor market. The results are consistent with preferences and psychological explanations for the underrepresentation of women in math-intensive STEM fields.