Molecular diffusion plays an important role in many biologic phenomena. The ability to study diffusion, therefore, is extremely useful in physiology and medicine. MRI offers a non-invasive window to diffusion, particularly water self-diffusion. MRI techniques, which provide diffusion sensitivity or quantitation (diffusion tensor MRI [DTI]), have found widespread application in neuroscience and medicine, including the evaluation of stroke, brain development, tumor imaging, and demyelinating disorders. We discuss the tensor nature of diffusion and provide an overview of how DTI offers unique information on tissue organization, water mobility, and disease states, particularly those of neuro-ophthalmologic interest.