Obesity in adults and children is increasingly becoming a major health problem worldwide. However, the precise biological mechanisms governing this disease have not been fully elucidated. Obesity involves the complex interaction of a wide range of environmental and genetic factors. Additionally, there is now a growing body of evidence suggesting that alterations in metabolic environment during important periods of organ development can predispose individuals to later development of obesity and diabetes. Maternal obesity or malnutrition during pregnancy increases the risk for metabolic disorders (including obesity) in the offspring. Similarly, early postnatal overnutrition also predisposes offspring to adult obesity. The hypothalamus appears to play an essential role in controlling appetite. It undergoes a tremendous growth beginning early in gestation and continuing during the postnatal period. These developmental windows represent periods of sensitivity for hypothalamic development during which alterations in the nutritional and/or hormonal environment may perturb hypothalamic development and subsequent function.