Young people can be agents of sustainable change. To this end, environmental education programs aim to promote their environmental knowledge, attitudes, intentions, and behaviors. We synthesized five decades of research into the effectiveness of environmental education for children and adolescents. We searched PsycINFO, ERIC, and Scopus and identified 169 studies (512 effect sizes; 176,007 participants) conducted in 43 countries, across 6 continents. Environmental education significantly improved environmental knowledge (g = 0.953), attitudes (g = 0.384), intentions (g = 0.256), and—mostly self-reported—behavior (g = 0.410). Heterogeneity in effect sizes was substantial; none of the tested moderators (including participant age, educational approach, and study design) accounted for this variance. Our findings demonstrate the potential for environmental education to improve students’ environmental knowledge, attitudes, intentions, and behavior. They also reveal methodological challenges for the field. Future research priorities include identifying effective environmental education components and approaches.