We present the Mastery Grids system, an intelligent interface for online learning content that combines open learner modeling (OLM) and social comparison features. We grounded the design of Mastery Grids in self-regulated learning and learning motivation theories, as well as in our past work in social comparison, OLM, and adaptive navigation support. The force behind the interface is the combination of adaptive navigation functionality with the mastery-oriented aspects of OLM and the performance-oriented aspects of social comparison. We examined different configurations of Mastery Grids in two classroom studies and report the results of analysis of log data and survey responses. The results show how Mastery Grids interacts with different factors, like gender and achievement-goal orientation, and ultimately, its impact on student engagement, performance, and motivation.