Obesity is a chronic metabolic disorder characterized by the presence of a positive energy balance over time that leads to a deposit of fat considered to be excessive with regard to that considered normal. Obesity is more often associated with medical and surgical complications than in people with a normal body weight. Its prevalence is increasing in the Western world and it is one of the main public health problems. In the last years there have been important advances in the understanding of the genetic factors related to obesity. However, there are other factors of an environmental, cultural, hormonal, or psychological origin at the root of this problem. In most cases the diagnosis of obesity is obvious, but before beginning a treatment it is necessary to carry out an individual assessment of the characteristics of the clinical history, physical examination, and laboratory data. Reasonable objectives for weight loss should be established in function of the age, complications, and previous attempts at loosing weight. The treatment measures basically consist of adequating the nutrition to the individual's needs, increasing the physical activity, and changing the life habits. A pharmacological or surgical treatment may be a support measure to these measures.