In this article we take a longitudinal perspective to analyze gender differences in academic career attainment. We improve upon prior research both theoretically and methodologically. Theoretically, we incorporate the extent of research specialization as a form of professional capital that potentially improves productivity and visibility, especially for men. Methodologically, we introduce a measure of research specialization and examine how men's and women's productivity and visibility unfold over time, rather than examining discrete periods of scholars' careers. We find that specializing is most beneficial with regard to productivity--that is, it helps scholars publish more. Specialization's effects on visibility are less general: surprisingly, only women's visibility seems to benefit from specialization. We discuss these findings and their implications, and suggest avenues for future research on this important and newly recognized form of capital. (Contains 6 tables, 4 figures, and 14 notes.)