Diffusion-weighted MR imaging (DWI) is a technique in which microscopic water motion is responsible for the contrast within the image. Diffusion of water molecules alters conventional T1- and T2-weighted MR imaging, because it induces a signal dephasing and a signal loss. Clinical practice uses different representations of the results of DWI data processing: diffusion weighted images, DWI trace and ADC maps, which are all equivalent. DWI is more accurate than CT in localizing ischemic lesions shortly after stroke onset. DWI can show small lesions adjacent to the cerebrospinal fluid. The NINDS and ECASS studies have demonstrated an increased risk of hemorrhagic transformation in stroke patients with a large area of hypodensity on admission CT when treated with thrombolytic therapy. DWI affords accurate localization of strokes. Finally, DWI can more frequently differentiate a small deep subcortical infarct from a cortical or a combined cortical/subcortical lesion than conventional MR imaging can.