The diagnosis of acute cerebral ischemia remains difficult. Computed tomography and magnetic resonance tomography are not specific at early time points. Diffusion imaging a new technique for magnetic resonance imaging was introduced in experimental studies and shown to have much earlier sensitivity for acute ischemic infarction. Diffusion imaging demonstrated early hyperintensities within minutes after ischemic onset and correlated well with post-mortem studies of the final infarct size. On the basis of molecular motion of water, hyperintensities during ischemia on diffusion imaging are most likely related to the development of ischemia induced cytotoxic edema and to slower diffusional motion. Experimental treatment and reperfusion studies with diffusion imaging demonstrated lesion reversal in vivo. Continuous monitoring after induction of ischemia enables an in vivo demonstration of the evolution of the ischemic lesion. Quantitative analysis of the apparent diffusion coefficient may enable the differentiation of infarcted tissue in the core of the lesion from reversibly damaged tissue. Preliminary clinical studies demonstrated a superior sensitivity of diffusion imaging compared with T2-weighted imaging. Clinicians may be able in the near future to image early stroke and to assess therapy when the stroke is still evolving.