This lecture focuses on a contemporary societal problem, the obesity epidemic in the US and other countries, to illustrate how certain core principles of environmental psychology—including ecological and interdisciplinary analyses of people’s relations with their surroundings, can be applied to better understand complex social and environmental problems. Multiple environmental contributors to the obesity crisis in the US are discussed. Some of these etiologic factors are rooted in the design of our physical environments whereas others reside in the power of social networks to influence our behavior and well-being. In keeping with the action research orientation of environmental psychology, ecological and interdisciplinary analyses of contributors to the obesity crisis provide a foundation for establishing evidence-based public policies and community interventions to improve environmental quality and public health.