Journal of Medical Internet Research - International Scientific Journal for Medical Research, Information and Communication on the Internet #Preprint #PeerReviewMe: Warning: This is a unreviewed preprint. Readers are warned that the document has not been peer-reviewed by expert/patient reviewers or an academic editor, may contain misleading claims, and is likely to undergo changes before final publication, if accepted, or may have been rejected/withdrawn. Readers with interest and expertise are encouraged to sign up as peer-reviewer, if the paper is within an open peer-review period. Please cite this preprint only for review purposes or for grant applications and CVs (if you are the author). Background: As mobile technology continues expanding, researchers have been using mobile phones to conduct health interventions (mHealth interventions). The multiple features of mobile devices offer great opportunities to disseminate large-scale, cost-efficient, and tailored messages to participants. However, the interventions to date have shown mixed results, with a large variance of effect sizes (Cohen’s d = -.62 to 1.65). Objective: To generate cumulative knowledge that informs mHealth intervention research, the aims of the current study are twofold: (a) to calculate an overall effect magnitude for mHealth interventions compared to alternative interventions/conditions and (b) to analyze potential moderators of mHealth interventions’ comparative efficacy. Methods: Comprehensive searches of the Communication & Mass Media Complete, PsycINFO, Web of Knowledge, Academic Search Premier, PubMed and Medline databases were conducted to identify potentially eligible studies in peer-reviewed journals and conference proceedings as well as dissertations and theses. Search queries were formulated using a combination of search terms: “intervention” (Title/Abstract) AND “health” (Title/Abstract) AND “*phone*” OR “black-berr*” (OR mHealth OR “application*” OR app* OR mobile OR cellular OR “short messag*” OR palm* OR iPhone* OR MP3* OR MP4* OR iPod*) (Title/Abstract). Results: The search resulted in 3,424 potential studies, the abstracts (and full text, as necessary) of which were reviewed for relevance. Studies were screened in multiple stages using explicit inclusion and exclusion criteria, and citations were evaluated for inclusion of qualified studies. A total of 61 studies were included in the current meta-analysis. Results showed that mHealth interventions are relatively more effective than comparison interventions/conditions, with a small but significant overall weighted effect size (d = 0.31). Additionally, the effects of interventions are moderated by theoretical paradigm, three engagement types (i.e., changing personal environment, reinforcement tracking, social presentation), mobile use type, intervention channel, and length of follow-up. Conclusions: To the best of our knowledge, this is the most comprehensive meta-analysis to date that examined the overall effectiveness of mHealth interventions across health topics and is the first study that statistically tested moderators. Our findings not only shed light on intervention design using mobile devices, but also provide new directions for research in health communication and promotion using new media. Future scholarship is needed to examine the effectiveness of mHealth interventions across various health issues, especially those that have not yet been investigated (e.g., substance use, sexual health), engaging participants using social features on mobile devices, and designing tailored mHealth interventions for diverse subpopulations to maximize effects.