In 1995, the average American surgeon earned over $269,000 while family practice doctors earned $131,200. Using data from the Survey of Young Physicians and the American Medical Association's Socio-Economic Monitoring Survey, I find that only half of income differences between generalists and specialists can be explained by hours of work, residency training length, and observed and unobserved ability differences -- the three competitive market explanations for differences in doctors' incomes across specialties. The most likely explanation for the remaining income differences is differential entry barriers across specialties.