Twin studies have made significant contributions to our understanding of the roles of nature, nurture, and their mutual interactions in the development of obesity. Individual differences in body mass index (BMI) and other indicators of obesity are influenced by both genetic and environmental factors. Eating behavior is considered the primary factor that mediates the effects of genes on obesity. Individuals who are most genetically susceptible to obesity are likely to have the strongest reaction to the obesogenic environment. This explains why environmental factors strongly influence the differences in obesity prevalence between populations, but still the heritability of obesity remains high within populations. However, the relative importance of genetic factors, environmental factors shared by co-twins, and environmental factors unique to each twin individual vary over the human life course. This reflects significant changes in the social environment interacting with the genotype. The environment surrounding the individual changes throughout the human lifespan, and thus a life course approach is important in studies of obesity and interventions aimed at preventing it. The interaction between genetic susceptibility and the surrounding environment behind the development of obesity emphasizes the need to target interventions not only to individuals but also to wider environments.