This report reviews the evidence on how teacher professional development affects student achievement. Of over 1,300 studies, only nine meet the What Works Clearinghouse evidence standards, highlighting the scarcity of rigorous studies in this area. The report finds that substantial professional development (averaging 49 hours) can boost student achievement by about 21 percentile points. The effect size was consistent across mathematics, science, and reading/English language arts. All nine studies focused on elementary school teachers, with half on lower grades (kindergarten and first grade) and half on upper grades (fourth and fifth grades). Six studies were published in peer-reviewed journals, and three were unpublished doctoral dissertations. Five studies were randomized controlled trials, and four met evidence standards with reservations. Studies with more than 14 hours of professional development showed a positive and significant effect on student achievement, while those with less than 14 hours showed no significant effects. All studies used workshops or summer institutes, with follow-up sessions in most cases. The report emphasizes the need for better evaluation methods and suggests that future studies should address the direct and indirect effects of professional development on teachers and students.This report reviews the evidence on how teacher professional development affects student achievement. Of over 1,300 studies, only nine meet the What Works Clearinghouse evidence standards, highlighting the scarcity of rigorous studies in this area. The report finds that substantial professional development (averaging 49 hours) can boost student achievement by about 21 percentile points. The effect size was consistent across mathematics, science, and reading/English language arts. All nine studies focused on elementary school teachers, with half on lower grades (kindergarten and first grade) and half on upper grades (fourth and fifth grades). Six studies were published in peer-reviewed journals, and three were unpublished doctoral dissertations. Five studies were randomized controlled trials, and four met evidence standards with reservations. Studies with more than 14 hours of professional development showed a positive and significant effect on student achievement, while those with less than 14 hours showed no significant effects. All studies used workshops or summer institutes, with follow-up sessions in most cases. The report emphasizes the need for better evaluation methods and suggests that future studies should address the direct and indirect effects of professional development on teachers and students.