This longitudinal study of middle school science teachers explored if a teacher participation in the TPD program resulted in change in instructional practice as well as a significant increase in student learning. Four participating schools were matched and randomly assigned to intervention and control groups. Teacher and student outcomes were compared. Eight teachers from Bryce and Zion Middle Schools participated in the 2-week summer institute, followed by monthly release day professional development sessions focused on implementing instruction outlined in the National Science Education Standards. Student achievement was assessed using the pre- or postinstruments. Students of teachers at treatment schools experienced significantly larger gains than students at the control schools. TPD intervention teachers experienced increase in teaching effectiveness. Findings in this study revealed the positive impact that whole-school, sustained, collaborative, professional development programs have on improving teacher practice and student achievement at the school level. (Contains 6 figures and 3 tables.)