We present evidence that is consistent with large disparities across firms in their on-the-job learning opportunities using administrative datasets from Brazil and Italy. We categorize firms into discrete “classes”—which our conceptual framework interprets as skill-learning classes—using a clustering methodology that groups together firms with similar distributions of unexplained wage growth. Mincerian returns to experience vary widely across experiences acquired in different firm classes. Four tests leveraging firm movers, occupation/industry switchers, hiring wages, and displaced workers point toward a portable and general human capital interpretation. Heterogeneous employment experiences explain an important share of wage variance by age 35.