This report reviews the evidence on how teacher professional development affects student achievement. Over 1,300 studies were examined, but only nine met What Works Clearinghouse evidence standards, indicating a lack of rigorous studies directly examining this link. The report finds that teachers who receive substantial professional development—on average 49 hours in the nine studies—can boost their students' achievement by about 21 percentile points. The connection between teacher professional development and student achievement is intuitive, but demonstrating it is challenging. The nine studies that met evidence standards focused on elementary school teachers and their students, with about half focusing on lower elementary grades (kindergarten and first grade) and the other half on upper elementary grades (fourth and fifth grades). Six studies were published in peer-reviewed journals, while three were unpublished doctoral dissertations. The studies ranged from 1986 to 2003. Five studies were randomized controlled trials that met evidence standards without reservations, while four met standards with reservations. Four studies focused on student achievement in reading and English/language arts, two on mathematics, two on mathematics and reading and English/language arts, one on science, and one on mathematics, science, and reading and English/language arts. Only one effect of the 20 identified across the nine studies was negative, and only one effect was zero. The other 18 were positive. Studies with more than 14 hours of professional development showed a positive and significant effect on student achievement, while the three studies with the least amount of professional development (5–14 hours total) showed no statistically significant effects. All nine studies employed workshops or summer institutes, with follow-up sessions supporting the main professional development event in all but one study. Professional development went directly to teachers rather than through a "train-the-trainer" approach. The report highlights the problems of many studies of professional development, emphasizing the need for researchers to avoid methodological pitfalls. Future studies should focus on the direct effect of professional development on teachers and its indirect effect on students. The report also notes that the lack of variability in form and the great variability in duration and intensity across the nine studies makes it difficult to discern patterns in these characteristics and their effects on student achievement. A larger number of rigorous studies would be needed to determine whether intensive, sustained, and content-focused professional development is more effective.This report reviews the evidence on how teacher professional development affects student achievement. Over 1,300 studies were examined, but only nine met What Works Clearinghouse evidence standards, indicating a lack of rigorous studies directly examining this link. The report finds that teachers who receive substantial professional development—on average 49 hours in the nine studies—can boost their students' achievement by about 21 percentile points. The connection between teacher professional development and student achievement is intuitive, but demonstrating it is challenging. The nine studies that met evidence standards focused on elementary school teachers and their students, with about half focusing on lower elementary grades (kindergarten and first grade) and the other half on upper elementary grades (fourth and fifth grades). Six studies were published in peer-reviewed journals, while three were unpublished doctoral dissertations. The studies ranged from 1986 to 2003. Five studies were randomized controlled trials that met evidence standards without reservations, while four met standards with reservations. Four studies focused on student achievement in reading and English/language arts, two on mathematics, two on mathematics and reading and English/language arts, one on science, and one on mathematics, science, and reading and English/language arts. Only one effect of the 20 identified across the nine studies was negative, and only one effect was zero. The other 18 were positive. Studies with more than 14 hours of professional development showed a positive and significant effect on student achievement, while the three studies with the least amount of professional development (5–14 hours total) showed no statistically significant effects. All nine studies employed workshops or summer institutes, with follow-up sessions supporting the main professional development event in all but one study. Professional development went directly to teachers rather than through a "train-the-trainer" approach. The report highlights the problems of many studies of professional development, emphasizing the need for researchers to avoid methodological pitfalls. Future studies should focus on the direct effect of professional development on teachers and its indirect effect on students. The report also notes that the lack of variability in form and the great variability in duration and intensity across the nine studies makes it difficult to discern patterns in these characteristics and their effects on student achievement. A larger number of rigorous studies would be needed to determine whether intensive, sustained, and content-focused professional development is more effective.