Diffusion MR imaging provides a novel way to characterize tissues based on sensitivity to the microscope molecular motion of water. Clinical implementation requires strong, fast hardware and careful post-processing of diffusion parameters. It is important to recognize that diffusion images and derivatives such as the trace of the diffusion tensor are quite specific in reflecting the physical properties of diffusion, but are non-specific for pathology. Restricted diffusion is the earliest clinically detectable sign of ischemia, but similar diffusion changes can be seen with infection and some tumors. Diffusion MR techniques are providing new ways to study problems in oncology, epilepsy, white matter disorders, and infectious diseases, both for research and clinical applications.