The purpose of this study was to examine the sources of efficacy of experienced teachers who are making the transition to reform-based mathematics teaching, by examining the criteria that they use to evaluate their teaching effectiveness. The quantity and quality of 10 elementary teachers' verbal reflections on the success of their lessons were compared as a function of the length of time that they had participated in a professional development project. Results suggest that the sources of information upon which teachers base evaluations of their teaching change in important ways as teachers shift from traditional to reform-based mathematics teaching. Implications are discussed for understanding how teacher self-efficacy for reform-based teaching can be enhanced through professional development. (Contains 2 tables and 1 note.)