With the increasing number of older adults in the society, healthy aging has become a public health priority. Targeting older adults’ sedentary behavior seems to be a promising strategy to foster healthy aging. Unfortunately, only a limited number of mobile health (mHealth) interventions exist to reduce sedentary behavior in older adults. Nevertheless, recent evidence has shown that mHealth interventions are more effective in reducing sedentary behavior compared to traditional interventions. mHealth interventions usually are complex interventions, incorporating multiple behavior change techniques, adopting different modes of delivery, and including a wide range of design features. Due to this complex nature, a new perspective is needed to evaluate mHealth interventions focusing on what works for whom, under which circumstances and how. This project will offer this new perspective by applying a realist approach. The overall aim of the current project is to gain a deep understanding of how, why, and in which circumstances mHealth interventions aimed at the reduction of older adults’ sedentary behavior work. To do so, an initial program theory will be determined based on a realist synthesis. Subsequently, this program theory will be used to develop an mHealth intervention aimed at the reduction of older adults’ sedentary behavior. Finally, the mHealth intervention will be assessed using a realist evaluation, and a final program theory will be established.