Presently, obesity constitutes a serious problem of public health in Chile and in many others countries, not only because of its high prevalence, but also by its influence in the production of chronic non transmissible illnesses of the adult. Nowadays, it is well known that the explanation for the high prevalence of obesity is not only the lack of relationship between caloric intake and expenditure, but there are many others possible etiologies that are analyzed in this article. Also, many questions related to the production of obesity are attempted to be explained, such as why the epidemic in the recent decades, why it stabilized, why affect children and why it affects particularly poor people. The assay finishes making a prognosis of how the epidemic of obesity will continue.