Inclusive education is a reform aimed at educating all students in general classrooms, independent of diversity features such as special educational needs, giftedness, or migration. Its successful implementation requires teachers with professional knowledge about inclusive education, skills to address the diverse needs in the classroom, and positive beliefs toward inclusive education. Teachers are provided with professional development opportunities, but are these effective in improving their learning process and positively impacting students' behavior? We conducted a meta-analysis to address this question. The screening of 12,050 search results revealed 342 eligible studies with more than 155,000 participants and 1123 effects from four outcome categories: teachers' knowledge, skills, and beliefs and students' behavior. We observed positive, though varying, effects on all four outcome categories: large effects on teachers' knowledge regarding inclusive education (g = 0.93), moderate effects on teachers' skills (g = 0.49), small effects on teachers' beliefs (g = 0.23), and small-to-moderate effects on student behavior (g = 0.37). We also examined factors that might explain the differences in the strength of training effects. The data suggest that long-term training with high practical relevance and active learning opportunities facilitates transfer to schools.