In this article we explore the factors associated with women’s choices to pursue Science, Technology, Engineering, and Mathematics (STEM) careers. Two 20-year longitudinal studies that were conducted in the United States surveyed adolescents on their educational and career aspirations. Interviews were conducted with a special subset of women when they were in their mid-30s and 40s to understand their decisions to (1) aspire to STEM careers if they initially had non-STEM career aspirations, or (2) leave or not pursue a STEM career if they initially had STEM career aspirations. Findings from semi-structured interviews uncovered three themes that participants used in explaining their career decisions: (1) the importance of family and work/life balance; (2) the importance of teachers and classroom experiences; and (3) interest in, and perceived value of, STEM subjects. We discuss the implications of these findings in relation to initiatives to encourage more women to pursue and persist in STEM careers.