Human capital-based theories of cities suggest that large, economically diverse urban agglomerations increase worker productivity by increasing the rate at which individuals acquire skills. One largely unexplored implication of this theory is that workers in big cities should see faster growth in their earnings over time than comparable workers in smaller markets. This paper examines this implication using data on a sample of young male workers drawn from the National Longitudinal Survey of Youth 1979 Cohort. The results suggest that earnings growth does tend to be faster in large, economically diverse local labor markets - defined as counties and metropolitan areas - than in smaller, more specialized markets. Yet, when examined in greater detail, I also find that this association tends to be the product of faster wage growth due to job changes rather than faster wage growth experienced while on a particular job. This result is consistent with the idea that cities enhance worker productivity through a job search and matching process and, thus, that an important aspect of 'learning' in cities may involve individuals learning about what they do well.