The data reviewed here emphasize the suggestion that obesity in children is the result of an interaction between a susceptible host and an environment that promotes the disease. A variety of environmental factors are related to childhood obesity. Nonetheless, except for television viewing, the behavioral correlates of the environmental associations remain unspecified. The goal for the coming decade is the identification of these behaviors, and their modification. The last five years have witnessed a dramatic increase in our understanding of the components of energy expenditure, and their relationship to the development of obesity. Both the early studies of Griffith and Payne, and the more recent studies of infants by Roberts suggest that reduced energy spent on activity may be the constitutional feature that accounts for an increased susceptibility to the disease. Reduced energy spent on activity may also account for some of the epidemiologic correlates. For example, differences in activity may account for the regional, seasonal, and population density effects on obesity, as well as some of the family line variables such as parental obesity, and family size. All represent promising sources for future investigation.