The study examined whether 7th-graders' knowledge of rational numbers improved when the students' math teachers participated in related professional development activities. The study analyzed data on about 4,500 students and 200 teachers from approximately 80 schools in 12 districts during the 2007-08 academic year. The study found that students in schools where teachers were offered extensive professional development by the study performed no better on a test of math achievement in rational numbers than students in comparison schools at the end of the 2007-08 academic year. However, the study found a significant positive impact of the professional development on one of the three measures of teacher instructional practices examined. Teachers who were offered the study's extensive professional development engaged in 1.03 more activities per hour that elicited student thinking than teachers not offered the study's professional development. The research described in this report is consistent with What Works Clearinghouse (WWC) evidence standards. The study was a well-implemented randomized controlled trial. [The following study is reviewed in this quick review: Garet, M., Wayne, A., Stancavage, F., Taylor, J., Walters, K., Song, M., et al. Doolittle, F. (2010). "Middle School Mathematics Professional Development Impact Study: Findings after the first year of implementation" (NCEE 2010-4009). Washington, DC: National Center for Education Evaluation and Regional Assistance, Institute of Education Sciences, U.S. Department of Education. This report may be found at ED509306.]