We designed a professional development (PD) program to increase the teacher efficacy of mathematics teachers. We randomly assigned 106 Grade 6 teachers in 1 school district to treatment and control conditions in a delayed-treatment design. The PD explicitly addressed 4 sources of teacher-efficacy information identified in social-cognition theory (Bandura, 1997). Treatment teachers outperformed control-group teachers on 3 measures of teacher efficacy, but results were statistically significant only for efficacy for classroom management. We attributed the teacher-efficacy effects of the PD (6% of the variance) to the priority given in the PD to management of classroom discussions and overt attempts by PD leaders to redefine teacher conceptions of classroom success. (Contains 1 figure, 4 tables, and 2 notes.)