Brain diffusion tensor (DT) MR imaging is a noninvasive quantitative method for mapping the diffusivity of free-water molecules. Quantitative values and graphical maps including neural fiber tractogram derived from DT MR imaging are promising approaches to evaluate white-matter integrity. The authors address technical issues of DT MR imaging and present clinical cases including X-linked adrenoleukodystrophy, cerebral palsy, and holoprosencephaly. Mouse experiments also are introduced in this article.