Background: Numerous smartphone applications (app) are targeting physical activity (PA) and/or healthy eating (HE), but empirical evidence on their effectiveness for initialization and maintenance of behavior change, especially in children and adolescents is still limited. Individuals’ behavior is embedded in social settings, therefore, core settings such as the family need to be considered when designing mobile health (mHealth) apps. Objective: Purpose of this study therefore is to conceptualize a theory- and evidence-based mHealth intervention (called SMARTFAMILY (SF)) targeting PA and HE in a collective family-based setting. Subsequently, the app will be refined and re-evaluated in order to analyze additional effects of just-in-time adaptive interventions (JITAI) and gamification features. Methods: A smartphone app based on behavior change theories and -techniques was developed, implemented and evaluated in a collective family setting with family members individually and cooperatively using the SF-app (SF trial). Subsequently, existing evidence and gained results will be used to refine and -evaluate the app (SF2.0 trial). For both trials, a cluster-randomized controlled trial including three measurement occasions is conducted. ... mehrThe intervention group (IG) is using the app for three consecutive weeks, whereas the control group (CG) receives no treatment. Baseline- (T0) and post-measurements (T1) include PA (self-reported and accelerometry) and HE measurement (self-reported fruit and vegetable intake (FVI)) as primary outcomes. Secondary outcomes (self-reported) are intrinsic motivation, behavior-specific self-efficacy and the Family Health Climate (FHC), complemented by an intentional measure in SF2.0. Four weeks following T1, a follow-up assessment (T2) is completed by participants, consisting of all questionnaire items to assess the stability of the intervention effects. Mixed-method ANCOVAs are used to calculate primary intervention effects (PA, FVI) while controlling for covariates including FHC, behavior-specific self-efficacy, and intrinsic motivation. Results: A greater increase in PA (steps, minutes of moderate-to-vigorous PA) and HE is expected in the IG compared to the CG (SF and SF2.0). Furthermore, a further increase is expected in SF2.0 due to the additional motivating features. Conclusion: This study aims at creating and testing a theory- and evidence-based mHealth intervention targeting PA and HE in a collective family-based setting. It is hypothesized that targeting the whole family facilitates behaviour change on the individual and the family level as the implemented strategies address changes in daily family life. Furthermore, subsequent app development (SF2.0) with supplementary addition of motivation-enhancing features and a JITAI approach is expected to enhance positive intervention effects.