This study investigated the impact of a dynamic geometry (DG)-centered teacher professional development program on high school geometry teachers' content knowledge and their students' geometry learning. 64 geometry teachers were randomly assigned to an experimental (DG) group and a control group. Both groups received appropriate and relevant professional development. Classroom observation data and the teachers' responses to the implementation questionnaires revealed that most teachers in the DG group were faithful to the DG instructional approach. Teachers in the DG group scored higher on a conjecturing-proving test than did teachers in the control group. The students of teachers in the DG group scored significantly higher than the students of teachers in the control group on a geometry achievement test. [For the complete proceedings, see ED583989.]