The focus of this research was an evaluation of the impact of teacher professional development (PD) on student achievement during implementation of a reform curriculum. The PD consisted of five four-hour workshop sessions distributed over the time teachers were implementing the reform curriculum in their classrooms. The research was conducted in a mid-size, urban school district over the span of two years. Three groups of teachers were contrasted: teachers who continued to use the established curriculum (N = 5), teachers who implemented the reform curriculum without participating in the PD sessions (N = 5), and teachers who implemented the reform curriculum while participating in the PD sessions (N = 13). Teachers who participated in the PD had approximately a one standard deviation advantage in their students' achievement over those who did not. We collected evidence of particular features of the PD that explained the differences in student achievement. The features included: distributing the workshops throughout the implementation; engaging teachers in an active learning process situated in the curriculum; and facilitating a collaborative community of teacher professionals. This study led us to believe that not only are the individual features of the PD important, but the combination of all three together is particularly powerful. (Contains 2 tables and 1 figure.)