Conventional magnetic resonance (MR) imaging is able to show pathology early on and to provide the radiologist with some degree of lesion characterization based on the relaxation time of different tissues. Many times, however, conventional MR imaging is not capable of depicting abnormalities at a time when early therapy may be successful, or of differentiating among different types of lesions before surgery. Diffusion-weighted imaging (DWI), a technique that is relatively new, is rapidly gaining popularity. Its increased use stems from the fact that many of the newer MR units are echo-planar capable. Although DWI may be obtained without echo-planar techniques, most DWI is now obtained by using gradients capable of very fast rising times. Echo-planar DWI may be obtained in a matter of seconds and, thus, is much less sensitive to bulk motion than other imaging techniques. Although DWI has been used extensively for the evaluation of acute cerebral infarctions, new uses for it are being explored constantly. In this article we address the nature of DWI and its use in the stroke patient as well as in other clinical situations where we believe it is useful.