Psychiatric comorbidities of Parkinson's disease, multiple sclerosis, and seizure disorders include depression, anxiety, psychosis, and behavioral problems. They are common complications of both diagnosis and treatment. Shared symptoms of psychiatric comorbidity and neurological illness make identification of psychiatric illness challenging and suggest common underlying pathophysiology for some symptoms. Diagnosis and management of comorbid psychiatric illness should include consideration of multiple causes, including the neurological illness itself, primary psychiatric disorders, side effects of treatment, and other environmental and psychological factors. Treatment of neurological illness is a frequent cause of psychiatric comorbidity and complicates the management of comorbid anxiety, depression, and psychosis. Careful selection and dosing of psychotropic medication can avoid worsening neurological symptoms. Psychotherapy and social support provide effective adjunctive treatment.