The built environment, composed of all the buildings, spaces, and products that have been created or modified by people, has undeniably an impact on our health and quality of life. This first issue of the TOPO collection documents, in a nutshell, the relation between the built environment and physical activity amoung young people. It addresses the following questions: "What opportunities provided by the built environment promote physical activity by young people?", "What characteristics of the built environment promote a physically active lifestyle among young people?", "What…