Behavioral neurology has primarily focused on brain-behavior relations as revealed by disorders of the cerebral cortex and subcortical gray matter. Disorders of cerebral white matter have received less attention. This article considers the contribution of cerebral white matter to normal behavioral function and the effects of white matter disorders on behavior. Diffuse dysfunction is more common than focal impairment, and the term white matter dementia has been proposed as a clinical entity. Conventional neuroimaging has enabled more accurate identification of white matter regions participating in neurobehavioral operations, and newer imaging techniques may define white matter connectivity within and between the hemispheres. As an essential component of neural networks, cerebral white matter contributes to cognitive and emotional functions, and lesions of white matter disconnect these networks to produce neurobehavioral syndromes.