The definition and risks of obesity have been reviewed and a nomogram provided for reference. Organization of information about the syndromes of obesity has been approached from several points of view. An anatomic classification has been developed, in which generalized and localized forms of fat accumulation can be separated. Hypercellularity of the adipose tissue in the childhood-onset forms of obesity is usually, but not always, present. Etiologic mechanisms are also useful in classifying obesity. This nosologic approach has been derived largely from experimental studies but has contributed significantly to understanding of pathogenetic mechanisms in man. Hypothalamic obesity is now thought to result from augmented secretion of insulin. The recessively inherited forms of obesity, on the other hand, appear to result from loss of a thermogenic system involving the ouabain-suppressible thyroid-induced (Na+ + K+) -ATPase which, in turn, accounts for the myriad of defects in these animals. Techniques of cybernetic engineering provide a third approach to classification of the syndromes of obesity. The control of body fat was analyzed as an analogy to the control of temperature in a building. These various approaches, and the new insights which they have provided for understanding the syndromes of obesity, promise to provide new pathways for pharmacologic intervention in the treatment of this problem.