Learning by doing matters for professional investors. We develop a new methodology to show that mutual fund managers outperform by a risk-adjusted 1.5% per quarter in industries where they have experience. The key to our identification strategy is that we look "inside" funds and exploit heterogeneity in experience for the same manager at a given point in time across industries. As fund managers become more experienced, their trades become better predictors for abnormal stock returns around subsequent earnings announcements. Our approach identifies experience as a first-order driver of observed mutual fund manager skill.