This study aimed to assess the effectiveness of behavior change (BC) interventions designed to promote physical activity and healthy eating and investigate whether theoretically-specified BC techniques improve outcomes. A systematic review of 122 evaluations (N = 44,747) found an overall pooled effect size of 0.31 (95% CI 0.26 to 0.36) with 69% heterogeneity. The technique "self-monitoring" explained the greatest amount of heterogeneity (13%). Interventions combining self-monitoring with at least one other technique from control theory were significantly more effective (0.42 vs. 0.26). The study classified interventions according to BC techniques and used meta-regression to identify effective components. Results showed that self-monitoring and other self-regulation techniques significantly improved effectiveness. The study highlights the importance of self-regulation techniques in promoting physical activity and healthy eating. The findings suggest that interventions incorporating self-monitoring and other self-regulation techniques are more effective. The study also found that the number of BC techniques did not increase effectiveness. The study concludes that self-monitoring and other self-regulation techniques are important for effective interventions. The study recommends further research to test these findings in larger samples. The study provides evidence that self-monitoring and other self-regulation techniques are important for effective interventions. The study also found that the number of BC techniques did not increase effectiveness. The study concludes that self-monitoring and other self-regulation techniques are important for effective interventions. The study recommends further research to test these findings in larger samples.This study aimed to assess the effectiveness of behavior change (BC) interventions designed to promote physical activity and healthy eating and investigate whether theoretically-specified BC techniques improve outcomes. A systematic review of 122 evaluations (N = 44,747) found an overall pooled effect size of 0.31 (95% CI 0.26 to 0.36) with 69% heterogeneity. The technique "self-monitoring" explained the greatest amount of heterogeneity (13%). Interventions combining self-monitoring with at least one other technique from control theory were significantly more effective (0.42 vs. 0.26). The study classified interventions according to BC techniques and used meta-regression to identify effective components. Results showed that self-monitoring and other self-regulation techniques significantly improved effectiveness. The study highlights the importance of self-regulation techniques in promoting physical activity and healthy eating. The findings suggest that interventions incorporating self-monitoring and other self-regulation techniques are more effective. The study also found that the number of BC techniques did not increase effectiveness. The study concludes that self-monitoring and other self-regulation techniques are important for effective interventions. The study recommends further research to test these findings in larger samples. The study provides evidence that self-monitoring and other self-regulation techniques are important for effective interventions. The study also found that the number of BC techniques did not increase effectiveness. The study concludes that self-monitoring and other self-regulation techniques are important for effective interventions. The study recommends further research to test these findings in larger samples.