There have been major advancements in cardiac surgery over the past two decades and a concomitant decrease in mortality and major morbidity. The improved safety in cardiac procedures permitted 330,000 operations involving cardiopulmonary bypass in 1992. However, several recent studies have demonstrated that cardiac surgery poses substantial risk of negative neurologic and neuropsychologic outcomes. Although very few patients die as a result of a cardiac operation, more than two thirds of patients demonstrate evidence of neuropsychologic dysfunction postoperatively. The mechanisms contributing to neuropsychologic deficits after cardiopulmonary by-pass are uncertain. To characterize the incidence and severity of such deficits after cardiac operations, a concise battery of neuropsychologic tests that provides reliable evidence of subtle brain trauma is essential. With an objective, valid measure of brain injury, the etiology of neuropsychologic deficits can be identified and either eliminated or the effects ameliorated. The proper selection and use of neurobehavioral tools provides a basis to evaluate the efficacy of surgical and pharmacologic interventions to further improve neurologic outcome after cardiopulmonary bypass.