To provide practitioners with useful information about how to promote proenvironmental behavior (PEB), a meta-analysis was performed on 87 published reports containing 253 experimental treatments that measured an observed, not self-reported, behavioral outcome. Most studies combined multiple treatments, and this confounding precluded definitive conclusions about which individual treatments are most effective. Treatments that included cognitive dissonance, goal setting, social modeling, and prompts provided the overall largest effect sizes (Hedge’s g > 0.60). Further analyses indicated that different treatments have been more effective for certain behaviors. Although average effect sizes are based on small numbers of studies, effective combinations of treatments and behaviors are making it easy to recycle, setting goals for conserving gasoline, and modeling home energy conservation. The results also reveal several gaps in the literature that should guide further research, including both treatments and PEB that have not been tested.