A great deal of knowledge has been accumulated in recent years, concerning the aging human brain in health and disease, with the advent of newer methods of measuring cerebral blood flow (CBF) and metabolism. It is well documented that even in normal aging, the functional metabolism of the brain and its blood supply inevitably decline. This accounts for the widespread clinical observations that the young tolerate disorders of the brain better than do the elderly. It also accounts for difficulties in correctly diagnosing and treating cognitive disorders among the elderly. Neuronal and vascular reserves become progressively depleted during normal aging, so that one or more different disease processes may contribute to cognitive declines. Computed tomography (CT) densitometry, coupled with xenon-enhanced CT CBF measurements, provide a noninvasive method for separating the changes within cerebral cortex, subcortex, and white matter that occur in normal aging from those due to pathological abnormalities.