This study aimed to assess the effectiveness of active behavior change (BC) interventions designed to promote physical activity and healthy eating, and to investigate whether theoretically-specified BC techniques improve outcomes. The authors systematically reviewed 122 evaluations (N = 44,747) of interventions using behavioral and/or cognitive techniques to increase physical activity and healthy eating in adults. The intervention content was classified into 26 BC techniques, and the effects of individual techniques and a theoretically-derived combination of self-regulation techniques were assessed using meta-regression. The overall pooled effect size was 0.31 (95% CI 0.26 to 0.36), with moderate heterogeneity (I² = 69%). Self-monitoring explained the greatest amount of among-study heterogeneity (13%). Interventions that combined self-monitoring with at least one other technique derived from control theory were significantly more effective than other interventions (0.42 versus 0.26). The findings suggest that classifying interventions according to component techniques and theoretically-derived technique combinations, and conducting meta-regression, can identify effective components of interventions designed to increase physical activity and healthy eating.This study aimed to assess the effectiveness of active behavior change (BC) interventions designed to promote physical activity and healthy eating, and to investigate whether theoretically-specified BC techniques improve outcomes. The authors systematically reviewed 122 evaluations (N = 44,747) of interventions using behavioral and/or cognitive techniques to increase physical activity and healthy eating in adults. The intervention content was classified into 26 BC techniques, and the effects of individual techniques and a theoretically-derived combination of self-regulation techniques were assessed using meta-regression. The overall pooled effect size was 0.31 (95% CI 0.26 to 0.36), with moderate heterogeneity (I² = 69%). Self-monitoring explained the greatest amount of among-study heterogeneity (13%). Interventions that combined self-monitoring with at least one other technique derived from control theory were significantly more effective than other interventions (0.42 versus 0.26). The findings suggest that classifying interventions according to component techniques and theoretically-derived technique combinations, and conducting meta-regression, can identify effective components of interventions designed to increase physical activity and healthy eating.