Findings from a metasynthesis of 15 research reviews of in service professional development to improve or change teacher content knowledge and practice and student/child knowledge and behavior are described. The research reviews included 550 studies of more than 50,000 early intervention, preschool, elementary, and secondary education teachers, educators, and practitioners. The goal of metasynthesis is to identify the common and core features of in service professional development associated with changes and improvements in educator and student outcomes. In-service professional development experts' contentions about the key characteristics and core features of effective in-service training were used to code and analyze the research reviews. Results showed that in-service professional development was most effective when it included trainer introduction, demonstration, and explanation of the benefits of mastering content knowledge or practice, active and authentic teacher learning experiences, opportunities for teachers to reflect on their learning experiences, coach or mentor supports and feedback during the in-service training, extended follow-up supports to reinforce in-service learning, and in-service training and follow-up supports of sufficient duration and intensity to have discernible teacher and student effects. Implications for improving in-service professional development are described.