The effects of imprinted genes on fetal growth and development have been firmly established. By and large, their roles conform to a conflict over provision of limited maternal resources to offspring, such that paternally expressed imprinted genes in offspring generally promote growth of the fetus, while maternally expressed imprinted genes tend to restrict it. It is comparatively recently that the important effects of imprinted genes in postnatal physiology have begun to be demonstrated, although a similar conflict may apply. In this chapter, we shall review some of the genetic evidence for imprinted effects on obesity, consider the action of selected imprinted genes in the central and peripheral control of energy homeostasis and look in detail at the intriguing effects of imprinting at the Gnas locus. Finally, we shall discuss whether these observations fit expectations of the prevailing theory for the existence of imprinting in mammals and go on to consider imprinted genes as targets for developmental programming.