The Regional Educational Laboratory - Southwest (REL Southwest) conducted a systematic and comprehensive review of the research-based evidence on the effects of professional development (PD) on growth in student achievement in three core academic subjects (reading/ELA, mathematics, and science). The primary goal of this study was to address the question, What is the impact of teacher participation in professional development on student achievement? Nine studies emerged as meeting What Works Clearinghouse (WWC) evidence standards, from more than 1,300 manuscripts identified as potentially relevant. Although the number of studies that met evidence standards was small, the average overall effect size of 0.54 was observed when examined within the three content areas included in the review. The consistency of this effect size indicates that across all forms and content of PD, providing training to elementary school teachers does have a moderate effect on their students' achievement. However, because the average number of contact hours averaged almost 49 hours across the nine studies, the total contact hours must be substantial to get such an effect size. Because of the limited number of studies and the variability in the PD that was represented among the nine studies we examined, we were unable to make any conclusions about the effectiveness of specific PD programs or about the effectiveness of PD by form, content, or intensity. The following are appended: (1) Methodology; (2) Protocol for the review of research-based evidence on the effects of professional development on student achievement; (3) Key terms and definitions related to professional development; (4) List of keywords used in electronic searches; and (5) Relevant studies, listed by coding results. (Contains 3 boxes, 2 figures, 9 tables, and 3 notes.) [This report was produced for the National Center for Education Evaluation and Regional Assistance, Institute of Education Sciences (IES) by Regional EducationalLaboratory Southwest administered by Edvance Research.]