Teacher professional development is often proposed as a means for improving students' achievement; however, few studies have been successful in empirically connecting teacher inservice interventions to their students' achievement gains, and especially in mathematics. The research presents results of a longitudinal study of an inservice teacher professional development that had as its primary purpose to improve middle school students' mathematics achievement. The study utilized a cross-school comparative research approach for the purpose of examining students' math achievement trajectories Hierarchical linear modeling was used for the study's data analyses. Results of the study revealed that the teacher professional development intervention had positive impacts on both the participating teachers and their middle school students. The participating students' achievement improved significantly for those whose teachers participated in the teacher professional development intervention.