The pathophysiology of psychiatric disorders is complex and cannot be easily assessed by laboratory studies. Diffusion tensor imaging (DTI) is a magnetic resonance imaging technique increasingly used for noninvasive and quantitative evaluation of abnormalities of the cerebral white matter because it provides exquisite details on tissue microstructures and also can perform sophisticated computer-based analyses. We review basic principles of DTI; methods of diffusion tensor analysis; recent DTI findings in major psychiatric diseases, such as schizophrenia, bipolar disorder, major depressive disorder, and anxiety disorder; and factors to keep in mind when interpreting the results of DTI analysis. We recommend the frequent use of DTI as a routine clinical protocol to assess white matter abnormalities in patients with psychiatric disorders.