The nature of environmental influences on individual differences in weight and obesity is presently unclear. To resolve this issue, behavior genetic studies are reviewed for their relevance to environmental influences on weight and obesity. Results are consistent in suggesting that environmental experiences are important for weight and obesity, although they account for much less variation than do the effects of genes. Furthermore, only environmental experiences that are not shared among family members appear to be important. In contrast, experiences that are shared among family members appear largely irrelevant in determining individual differences in weight and obesity. These conclusions are consistent with a growing body of evidence on the relative unimportance of such shared experiences for many psychological characteristics.