This paper describes the effects of a professional development (PD) program--Developing Mathematical Thinking--on student achievement. Six Title I elementary schools with similar demographics, within one school district, were chosen to participate as either a treatment or comparison school. Three schools were chosen to participate in professional development that incorporates effective PD recommendations. All the teachers had to participate in all aspects of the PD, thereby eliminating potential self-selection bias. Using the state standardized achievement test as the before and after measure, results suggest improved student performance after professional development was implemented over a two year period.