Given growing interest in computing fields, as well as a longstanding gender gap in computer science, this study used nationwide survey data on college students during 4 decades to: (a) document trends in aspirations to major in computer science among undergraduate women and men; (b) explore the characteristics of women and men who choose to major in computer science and how this population has evolved over time; and (c) identify the key determinants of the gender gap in the selection of computer science majors during the past 4 decades. The data included 8 million students attending 1,225 baccalaureate-granting institutions from 1971 to 2011, with selected-year multivariate analyses of 18,830 computer science majors (and 904,307 students from all other majors). The results revealed heavy fluctuations in students' interest in computer science from 1971 to 2011, with trends highlighting a significant downturn between the late 1990s and 2011 as well as a persistent, sizeable underrepresentation of women across all years. The study also showed that while some of the traditional explanations for the gender gap in computer science held true, there have been distinctive shifts in who pursues computer science and why some students may be particularly interested in or dissuaded from the major.