The authors provide an overview of current clinical neuroimaging techniques for acute ischemic stroke. The pathophysiology of ischemic stroke is briefly reviewed, especially as it pertains to cerebral blood flow. There are three important goals that must be met in the imaging of an acute ischemic stroke: (1) to quantitate the extent of infarction already present; (2) to quantitate the amount of salvageable brain remaining; and (3) to identify the presence of arterial stenosis or occlusion that is both responsible for the stroke and potentially amenable to treatment. Methods of quantifying the degree of infarction, by magnetic resonance imaging or computed tomography, are compared. Techniques for measuring cerebral blood flow, to quantify tissue at risk, are detailed and compared. The quantification of cerebral blood flow is discussed in detail. Methods of visualizing the cerebral circulation are illustrated.