Principals are held accountable for student achievement although most studies find that they have no direct effect on it. In this study we tested a model hypothesizing that principals contribute to student achievement indirectly through teacher commitment and beliefs about their collective capacity. Path analysis of data from 205 elementary schools supported this hypothesis. Schools with higher levels of transformational leadership had higher collective teacher efficacy, greater teacher commitment to school mission, school community, and school-community partnerships, and higher student achievement. Increasing the transformational leadership practices in schools makes a small but practically important contribution to overall student achievement. (Contains 2 figures, 3 tables, and 3 notes.)