In this study of a sample of 148 teenagers from metropolitan Toronto, home range is found to be smaller in the city, in lower social classes, and-in the suburbs only-for girls and younger teenagers. Home range is the spatial manifestation of exploring the “fourth environment,” which is globally defined as the environment outside the home, playground, and specifically child-oriented institutions. The significance of this fourth environment in the process of growing up is discussed, and some findings are presented that indicate that home range may be related to use and knowledge of the environment. It is suggested that the local neighbour-hood continues to be an important developmental context for young people, and common interests of other population groups are recognized as enhancing the potential for planning that is responsive to children’s needs.