The number of women in science and engineering is growing, yet men continue to outnumber women, especially at the upper levels of these professions. In elementary, middle, and high school, girls and boys take math and science courses in roughly equal numbers, and about as many girls as boys leave high school prepared to pursue science and engineering majors in college. Yet fewer women than men pursue these majors. Among first-year college students, women are much less likely than men to say that they intend to major in science, technology, engineering, or math (STEM). By graduation, men outnumber women in nearly every science and engineering field, and in some, such as physics, engineering, and computer science, the difference is dramatic, with women earning only 20 percent of bachelor's degrees. Women's representation in science and engineering declines further at the graduate level and yet again in the transition to the workplace. Drawing on a large and diverse body of research, this report presents eight recent research findings that provide evidence that social and environmental factors contribute to the under-representation of women in science and engineering. (Contains 21 figures, 13 footnotes, and a bibliography.) [This report was supported by the Letitia Corum Memorial Fund, the AAUW Mooneen Lecce Giving Circle, and the Eleanor Roosevelt Fund.]