It is now clear that early experience influences the long-term development of behavioral, neuroendocrine, and cognitive systems in a number of animal species. This article examines the effects of early life stress on the development of the rodent. Postnatal maternal separation is often used as a potent early life stressor, and some of the major findings from these studies are discussed. A majority of these studies have shown that early life stress can lead to a heightened stress response when maternally deprived rodents are tested as adults. The effects of early life stress on the development of brain structures involved in regulating the stress response as adults are also discussed. Finally the influence of both genetics and maternal style are mentioned in relation to their ability to alter the effects of early life stress.