Engaging in sufficient physical activity has several beneficial effects on physical and mental health. Still, a large proportion of the Western population doesn't meet the guidelines of being moderately to vigorously active for a minimum of 30 minutes on at least five days a week. Mobile technology provides possibilities for personalized coaching systems, and the results of existing mobile interventions look promising. Here, the authors present a mobile system that goes beyond existing (mobile) physical activity interventions. The approach combines theory and evidence-based behavior-change techniques with a model-based reasoning system, to provide the right support and strategies at the right time for obtaining a physically active lifestyle.