This paper studies the gender gap in performance among associate lawyers in the United States. The legal profession traditionally uses measures of performance to reward lawyers, namely, the number of hours billed and the amount of new client revenue generated. We find evidence of a performance gap. Male lawyers bill 10% fewer hours billed and bring in more than double new client revenue. We do not find evidence of explicit discrimination; instead the presence of young children and differences in aspirations to be a law-firm partner account for a large part of the difference in performance. These results have important consequences for earnings. While individual and firm characteristics explain up to 50 percent of the gender earnings gap, performance measures explain most of the remaining part of the gap.