Despite considerable sparing of function following some forms of early brain disease, neural plasticity is far from complete. Many children with early brain insults, including those who sustain traumatic brain injury (TBI), are susceptible to both immediate and long-term neurobehavioral impairments. To introduce this symposium, the present article reviews existing research on the effects of 3 age-related factors on outcomes: age at injury, time since injury, and age at testing. Research findings support the hypothesis that development is more adversely affected the younger the child at the time of brain insult. Although we know less about how outcomes are related to the other developmental factors, there is little evidence that sequelae resolve with age. Potential brain mechanisms responsible for age-related differences are explored and methodological problems are considered. Emphasis is placed on the importance of prospective designs, measurement of developmental change, comprehensive assessments of outcome, and evaluation of factors contributing to variability in outcomes, such as premorbid status, type of brain injury, and environmental influences. Papers in this series demonstrate the utility of these methods and shed new light on developmental processes associated with childhood brain insults. (JINS, 1997, 3, 555–567.)