ABSTRACT The technological capabilities of mobile phones have made them a useful tool for delivering interventions, but additional research is needed to determine the mechanisms underlying the comparative effectiveness of mobile health interventions. This meta-analysis analyzes the relative effectiveness of mobile phone-based health interventions relative to comparison/control groups (e.g., eHealth interventions, standard of care, etc.), the utility of the theory of planned behavior in mobile phone-based health interventions, and the roles of various moderators. One hundred eighteen studies met inclusion criteria and contributed to an overall effect size of d = 0.27 (95% CI [.22, .32]). Findings indicate that mobile phone-based health interventions are significantly more effective than comparison/control conditions at improving health behaviors. Additionally, perceived behavioral control was a significant moderator providing some support for the usefulness of theory of planned behavior in mobile phone-based health interventions.