Abstract: Studying physicians in training, I investigate how uncertainty and tacit knowledge may give rise to "weak best practices," which allow for significant practice variation in organizations. Consistent with tacit learning, and empirically exploiting a discontinuity in the formation of teams, I find that relative experience substantially increases the influence of a physician on variation. Learning sufficient to generate convergence occurs for patients on services driven by specialists, a difference unexplained by formal diagnostic codes. In contrast, rich physician characteristics correlated with preferences and ability, and quasi-random assignments to high- or low-spending supervising physicians explain little if any variation.