The cognitive and behavioral disorders associated with cancer and its treatments can have a tremendous impact on patients' quality of life. Brain tumors, leptomeningeal disease, and paraneoplastic syndromes have all been shown to cause specific neurobehavioral abnormalities. In addition, cancer patients frequently develop cognitive and behavioral alterations during or after radiation therapy, chemotherapy, or immunotherapy. Although some impairments are acute and reversible, others may persist after the cessation of treatment or have a delayed onset. These neurobehavioral disorders can range from profound intellectual decline (dementia) to subtle deficits evident only on sensitive neuropsychological tests. Even mild cognitive impairments may compromise an individual's ability to return to work or other activities.