Self-assessment is a powerful technique for improving achievement. In this article we outline a theory of teacher change that links self-assessment by teachers to their professional growth. This theory provides avenues for peers and change agents to influence teacher practice. We apply the theory to change in mathematics teaching and report an explanatory case study in which use of the self-assessment tool, in combination with other elements, contributed to change in the instructional practice of a grade 8 mathematics teacher. Provision of a self-assessment tool contributed to teacher growth by: (1) influencing the teacher's definition of excellence in teaching and increasing his ability to recognize mastery experiences; (2) helping the teacher select improvement goals by providing him with clear standards of teaching, opportunities to find gaps between desired and actual practices, and a menu of options for action; (3) facilitating communication with the teacher's peer; and (4) increasing the influence of external change agents on teacher practice. The study argues that providing a self-assessment tool is a constructive strategy for improving the effectiveness of in-service provided it is bundled with other professional growth strategies: peer coaching, observation by external change agents, and focused input on teaching strategies.