Obesity is defined as increased body weight caused by excessive accumulation of fat. Due to a very long period of undernutrition in human history, the contemporary human body regulation mechanisms seem to be biased in favor of preserving fat rather to eliminate it. At the highest risk are populations that suddenly gained wealth. The shift from undernutrition to overnutrition has occurred in a very short time, in many population groups in less than in one generation. The increase of obesity prevalence observed in the 20th century continues until present and it appears this trend will further continue in almost all countries in the world. Contemporary prevalence of adult obesity is very high in USA (33% in both gender), in oil-rich Arabian countries (30% in males, 40 % in females) and in European Union (up to 25% in both gender). The aim of contemporary research is to understand the molecular and neural systems which the body uses to regulate its storage of energy in the form of fat and how these systems can become unbalanced, leading to obesity. In spite of discovery of new hormones (e.g. leptin produced in adipose tissue) and of new mechanisms, the prevention and treatment of obesity remains an open problem. Obesity is associated with an increased risk of numerous comorbidities such as type 2 diabetes, metabolic syndrome, hypertension, cardiovascular diseases and osteoarthritis. In USA the impact of obesity on mortality may have decreased over time, perhaps because of improvements in public health and medical care. New data from USA and China suggest the lowest all-cause mortality in persons with a body mass index, BMI between 23.0 and 27.0 (Fig. 6, Tab. 1, Ref. 29). Full Text (Free, PDF) www.bmj.sk.