Childhood obesity, although a preventable condition, remains a major global public health concern. Despite tremendous efforts, researchers and policymakers have been unable to turn the tide on children’s weight gain. In recent years, Health Geographers have increasingly acknowledged the role of place in determining children’s levels of extracurricular physical activity, thereby influencing their body shapes. This recognition has not, however, led to a full understanding of the triad connecting the built environment to children’s physical activity and body composition. My dissertation therefore aimed to fill this gap by comprehensively uncovering the dynamics at work in this triad. An explanatory sequential mixed-methods research design was adopted, combining the strengths of quantitative spatial epidemiology and the qualitative exploration of children’s context-specific lifeworlds in London. The integration of findings obtained through these different research lenses showed that the built environment was severely implicated in determining the body composition of young citizens. This effect, however, was not direct, as out-of-school activity emerged as the crucial pivot mediating the built environmental-body composition relation. Through numbers and narratives, the myriad ways in which the environment, activity and body shape interacted were unveiled. First, I demonstrated the need to disentangle extracurricular physical activity and body mass metrics into their prime components. Having done so, I established that active school travel constitutes a primary pathway in tackling the overweight and obesity epidemic, due to its fat-mass-reducing effect and close associations with the built environment. Integration of quantitative and qualitative evidence showed how proximity to school, traffic safety, the provision of safe and well-maintained pavements and crossroads, and parental perceptions were crucially involved in this relationship. Having contributed to the translation of these findings into policy and practice through concrete policy recommendations, this research constitutes a bold step towards the creation of activity-inciting, leptogenic environments for children.