It is now believed that the factors leading to massive obesity are a caricature of those which are responsible for common obesity. The lipid component of daily diet is as important as the amount of calories ingested. Putting on weight occurs only in subjects with metabolic abnormalities or with genetically determined neuro-hormonal anomalies. The weight increase tends to correct the initial metabolic disturbances, and in fact obesity is nothing but a phenomenon of adaptation. The development of molecular biology techniques should help us, in the near future, to understand these major physiopathological disorders.