High pedagogical quality in early childhood education and care (ECEC) is related to developmental outcomes in young children. This review summarizes findings from (quasi)-experimental studies that evaluated in-service training effects for ECEC professionals on external quality ratings and child development. The aggregation of findings at teacher level (including 36 studies with 2,891 teachers) revealed a medium in-service training effect on process quality (effect size [ES] = 0.68, SE = 0.07, p < .001). Furthermore, a subset of nine studies (including 486 teachers and 4,504 children) that provided data on both quality ratings and child development were analyzed, and they showed a small effect at child level (ES = 0.14; SE = 0.02, p < 0.001) and a medium effect at the corresponding classroom level (ES = 0.45, SE = 0.11, p < 0.001). Variance in effect sizes at child level was significantly related to in-service effects on quality ratings (53% explained variance). The results show that quality improvement is a key mechanism to accelerate the development of young children.