Poor diet including inadequate vegetable consumption is among the leading causes of death in the US and UK. Mobile applications (apps) have been heralded as a potentially transformative tool for delivering behavioural nutrition interventions at scale, but most have yet to incorporate theory-based strategies known to drive changes in health behaviours or undergo systematic testing to demonstrate their effectiveness. Moreover, published frameworks to guide the development of such technologies have yet to integrate best practices from academia and industry. The aim of this dissertation was therefore to introduce a process for guiding the development of more effective mobile interventions and to apply that process in the development and evaluation of a mobile application to increase vegetable consumption. This dissertation introduced IDEAS (Integrate, DEsign, Assess, Share), a step-by-step process for integrating behavioural theory, design thinking, and evaluation to guide the development of more effective mobile health interventions. IDEAS was then applied in the iterative development of Vegethon, a mobile app to increase vegetable consumption among overweight adults. Behavioural theory and two stages of qualitative interviews with participants (n=18; n=14) shaped intervention conception and refinement. The final mobile app enabled easy self-monitoring and incorporated 18 behaviour change techniques including goal setting, feedback, social comparison, prompts, framing, and identity. A pilot randomized controlled trial among overweight adults (n=17) was conducted and indicated the initial acceptance, feasibility, and efficacy of the intervention, showing significantly greater consumption of vegetables among the intervention vs. control condition after 12 weeks (adjusted mean difference: 7.4 servings; 95% CI: 1.4, 13.5; p=0.02). A more substantially powered randomized controlled trial among overweight adults (n=135) was conducted and similarly found significantly greater daily vegetable consumption in the intervention vs. control condition (adjusted mean difference: 2.0 servings; 95% CI: 0.2, 3.8, p=0.03). These findings show, for the first time in a rigorous randomized controlled trial, the efficacy of a stand-alone theory-based mobile app to increase vegetable consumption. Given the improved health outcomes associated with greater vegetable consumption, these data indicate the need for longer-term evaluations of Vegethon and similar technologies among overweight adults and other suitable target groups. Theory-based mobile apps may present a low-cost and readily scalable tool for delivering behavioural health interventions. The IDEAS framework may be useful to investigators in the development of their own mobile health interventions.