We examined the effect of different professional development programs on the accuracy of teachers' judgments of their students' learning during three academic years. Teachers participated in a program focused on improving (a) use of formative assessment, (b) student-centered mathematics instruction, (c) use of both formative assessment and student-centered mathematics instruction; or (d) neither—a control group. Teachers' judgment accuracy was greater for teachers who participated in professional development around improving student-centered mathematics instruction than for other groups. A multilevel analysis showed a significant positive relation between the accuracy of teachers' judgments and student achievement.