Rising concern about America's ability to maintain its competitive position in the global economy has renewed interest in science, technology, engineering and mathematics (STEM) education. To understand who enters into and completes undergraduate programs in STEM fields, this report examined data from three major national studies: the 1995-96 Beginning Postsecondary Students Longitudinal Study (BPS:96/01); the 2003-04 National Postsecondary Student Aid Study (NPSAS:04); and the Education Longitudinal Study of 2002/06 (ELS:02/06). STEM fields, as defined in this study, include mathematics, natural sciences (including physical sciences and biological/agricultural sciences), engineering/engineering technologies, and computer/information sciences. This study used students' reported major field of study to identify STEM entrants and considered a STEM entrant anyone who reported a major in a STEM field at any time during his or her postsecondary enrollment. Looking only at single points in time, STEM majors accounted for 14 percent of all undergraduates enrolled in U.S. postsecondary education in 2003-04 and 15 percent of 2003-04 high school graduates who were enrolled in postsecondary education in 2006. In general, the percentage of students entering STEM fields was higher among male students, younger and dependent students, Asian/Pacific Islander students, foreign students or those who spoke a language other than English as a child, and students with more advantaged family background characteristics and strong academic preparation than among their counterparts who did not have these characteristics. After 6 years of initial college enrollment, STEM entrants generally did better than non-STEM entrants in terms of bachelor's degree attainment and overall persistence. Although students in various STEM fields were generally alike in terms of their demographic, academic, and enrollment characteristics and their outcomes, those entering computer/information sciences differed in many respects. According to the BPS data, older students, students from low-income families, and those less academically prepared enrolled in computer/information sciences more often than did their peers who were younger, from high-income families, or more academically prepared. Additionally, compared to other STEM students, a larger percentage of computer/information sciences majors attended public 2-year institutions, enrolled in sub-baccalaureate programs, and attended classes exclusively part-time. A Technical Notes section describes data sources, study samples, weights, and derived variables used for this report. It also includes a crosswalk for the specific contents of the STEM categorization for various major fields of study. (Contains 2 figures and 8 tables.)