Obesity is a widespread condition, with different etiologies, that is usually treated only symptomatically i.e. through lowered energy intake. The existence of a latent situation of pre-obesity is postulated. The preobese is defined as a lean individual susceptible to easily develop obesity with unlimited food availability. The physiologic and metabolic pathways responsible of the appearance of obesity are revisited, as well as the current theories on body weight regulatory mechanisms. From this information, a classification of obesities is proposed: 1) Hypothalamic, 2) Bulimic, 3) Digestive, 4) Hyperinsulinemic, 5) Hypothermogenic, 6) Hypothyroid, and 7) Set point. These conditions should not be treated therapeutically in the same way, as the causes of development of the illness are not equal. The need to determine the individualized causes of obesity prior to any treatment is stressed.