While obesity is recognized as a major health risk factor, there is a lack of clear and consistent research results as to causes, course, or cure. One source of difficulty is the absence of a precise and consistent definition of “obesity,” a confusion between obesity, adiposity, or “fatness” as these might be defined either by medical criteria or by what is fashionable. A review of the diagnostic criteria, of the measures currently available, and their documented quality leads to a recommendation for the Body Mass Index (BMI) as the currently preferred measure of obesity in research studies, largely because of its simplicity relative to other measures. Ranges for ideal weights suggested in the other studies are summarized in terms of BMI in tabular form, and shown to be remarkably concordant.