Echo planar diffusion-weighted imaging (EP DWI) provides information about the physiologic state of the brain that is not available on conventional magnetic resonance (MR) images. Specifically, it provides signal proportional to the molecular diffusion of water molecules. It has proven highly sensitive in the detection of acute infarction and it is reliable in differentiating acute stroke from other diseases that mimic acute stroke clinically and on conventional MR images. With perfusion imaging, diffusion-weighted imaging is useful in predicting final infarct size and patient outcome. Diffusion MR is also becoming increasingly useful in the evaluation of a wide variety of other disease processes including neoplasms, intracranial infections and traumatic brain injury. Because acute stroke is common in the differential diagnosis of the majority of patients who present with acute neurologic deficits, diffusion-weighted imaging has become an essential sequence.