Obesity is a fundamental disorder of energy balance in which excessive energy stores accumulate in the form of fat in response to sustained high energy intake and/or low expenditure. While genetic factors inﬂuence obesity through endocrine mechanisms, the majority of endocrine changes observed in obese subjects are consequences of obesity. The endocrine mechanisms giving rise to disturbances of fat distribution, and by which obesity gives rise to its principal complications-diabetes, cardiovascular disease, and female reproductive dysfunction-are becoming clear. In this chapter we consider the unusual primary endocrine causes of obesity, including recently described genetic syndromes, and then focus on the more common alterations in endocrine function that are characteristic of obesity-disturbances in insulin secretion and action, adrenocortical function, sex steroid secretion, the growth hormone insulinlike growth factor and pituitary-thyroid axes. The evidence that these changes play a role in either the determination of corpulence or the perpetuation of the obese state is considered.