Family resemblances in weight status are well documented. Overweight parents are more likely to have overweight children than nonoverweight parents. Familial patterns of adiposity result from genes and family environmental factors working in concert, particularly for young children growing up within the family. Behavioral genetics research illustrates the important contribution of genetics to the obese phenotype, with genes or genetic similarity among family members explaining approximately 70% of the phenotypic variation in adiposity. Likewise, dramatic increase in the prevalence of overweight among children and adults within the past 20 year attest to the crucial role of the environmental in the development and maintenance of overweight. Environmental and genetic factors are unlikely to work independently. That is, individuals with a genetic propensity for overweight are likely to select environments for themselves and their children, including low levels of activity and high fat intake, that promote the development of overweight. This gene-environment correlation has not received adequate attention in obesity-related research. That is, although a great deal has been learned the genetics of obesity, relatively little information is available about environmental variables and intermediary behaviors that promote childhood overweight and mediate family resemblance in overweight. A research challenge for behavioral scientists is to delineate aspects of the family environment that mediate the development of family resemblances in adiposity and promote childhood overweight.