Diffusion tensor imaging (DTI) permits the quantitative evaluation of white matter pathology using measures of diffusion anisotropy. Fiber tractography based on DTI can reveal the three-dimensional white matter connectivity of the human brain. DTI fiber tractography is used to localize stroke lesions in relation to functionally important pathways and to assess wallerian degeneration, which may allow more accurate prognosis of long-term recovery or disability. DTI also improves the evaluation of hypoxic-ischemic injury to the developing brain of newborns and infants. DTI and fiber tractography may prove useful in elucidating alterations in brain connectivity resulting from neuroplasticity after stroke.