Teachers' belief systems about the inclusion of students with special needs may explain gaps between policy and practice. We investigated three inter-related aspects of teachers' belief systems: teachers' cognitive appraisals (e.g., attitudes), emotional appraisal (e.g., feelings), and self-efficacy (e.g., agency to teach inclusive classrooms). To date, research in this field has produced contradictory findings, resulting in a sparse understanding of why teachers differ in their belief systems about inclusive education, and how teachers' training experiences contribute to their development of professional beliefs. We used meta-analysis to describe the level and range of teachers' beliefs about inclusive education, and examine factors that contribute to variation in teachers' beliefs, namely (1) the point in teachers' career (pre-service versus in-service), (2) training in special versus regular education, and (3) the effects of specific programs and interventions. We reviewed 102 papers (2000-2020) resulting in 191 effect sizes based on research with 40,898 teachers in 40 countries. On average, teachers' cognitive appraisals, emotional appraisals, and efficacy about inclusion were found to be in the mid-range of scales, indicating room for growth. Self-efficacy beliefs were higher for preservice (M = 3.69) than for in-service teachers (M = 3.13). Teachers with special education training held more positive views about inclusion than regular education teachers (d = 0.41). Training and interventions related to improved cognitive appraisal (d = 0.63), emotional appraisal (d = 0.63), and self-efficacy toward inclusive practices (d = 0.93). The training was particularly effective in encouraging reflection of beliefs and, eventually, facilitating belief change when teachers gained practical experience in inclusive classrooms. Six key findings direct the next steps.