Positron emission tomography (PET) uniquely allows the in vivo regional measurement of several important physiologic parameters in living humans, including cerebral blood flow and oxygen metabolism. PET studies have advanced our understanding of normal human brain physiology and, as detailed in this article, our understanding of human cerebrovascular pathophysiology. This article focuses on knowledge gained from PET regarding acute ischemic stroke and chronic oligemia from arterial occlusive disease. Knowledge of the responses of the brain and its vasculature to ischemia and oligemia is growing more important with the increasing availability of CT and MR perfusion techniques.