Gender gaps in educational outcomes are a matter of real and growing concern. It has been known for a long time, since the 1970s, that girls outscore boys in the National Assessment of Educational Progress (NAEP) reading tests, while boys tend to outperform girls in math and science. Boys are increasingly less likely than girls to attend college and to receive a bachelor's degree. Female college students continue to be underrepresented in such technical fields as engineering and computer science. One popular, if controversial, response to these patterns has been a renewed push for single-sex education--an effort that has drawn support from across political divides. The majority of arguments for single-sex schools and classrooms focus on the effects on interactions among students, but they also present the possibility of greatly increasing the number of students with teachers of the same gender. Is there any convincing evidence that doing so could make a difference in education--for boys and girls alike? This article offers new and convincing evidence of the differential impact of a teacher's gender on student learning. The author investigated the effect of a teacher's gender using the National Education Longitudinal Survey (NELS), which contains data on a nationally representative sample of nearly 25,000 8th graders from 1988. In addition to examining the effect of teacher gender on students' test-score performance, he examined teacher perceptions of a student's performance and student perceptions of the subject taught by a particular teacher. Results confirm that a teacher's gender does have large effects on student test performance, teacher perceptions of students, and students' engagement with academic material. Simply put, girls have better educational outcomes when taught by women and boys are better off when taught by men. These findings persist, even after accounting for a variety of other characteristics of students, teachers, and classrooms that may influence student learning. (Contains 3 figures.)