There are about 250 million adults who are obese and at least 500 million who are overweight in the world at the moment (1). Given these numbers, it is not surprising that the genetics of human obesity is receiving increasing attention. The interest in the causes of the present epidemic of obesity in the Western world and the promise of ﬁnding new potentially prophylactic and therapeutic means are largely responsible for this new interest. Excess weight has also become the most important public health problem in the United States and Canada, and this has contributed enormously to the present interest for the molecular and genetic causes of the problem. Several lines of research are currently being explored in the eﬀort to identify the genes involved in causing obesity, rendering someone susceptible to obesity, or determining the metabolic response to an obese state.