Multiple sclerosis is associated with diverse behavioral changes. Rates of major depression approach 50% over the course of patients' lives, rates of bipolar disorder and psychosis are twice as common when compared to rates in the general population and pseudobulbar affect will afflict one in ten patients. However, these disorders often respond well to pharmacologic treatment and in the case of major depression, some forms of psychotherapy too. This article summarizes these mental state changes, their pathogenesis and treatment. Correct diagnosis and treatment can substantially improve the quality of life for MS patients.