Obese people, who are already subject to adverse health effects, are additionally victimized by a social stigma predicated on the Hippocratic nostrum that weight can be controlled by 'deciding' to eat less and exercise more. This simplistic notion is at odds with substantial scientific evidence illuminating a precise and powerful biologic system that maintains body weight within a relatively narrow range. Voluntary efforts to reduce weight are resisted by potent compensatory biologic responses. This article will review some of this evidence, together with promising avenues of research. Further progress in understanding and treating obesity will come not from repetition of anachronistic preconceptions but rather from the rigorous scientific approach that has driven advances in so many other areas of medicine.