The higher prevalence of psychiatric disorders in multiple sclerosis (MS) compared with the general population is well documented, with depression being the leading disorder. Apart from depression, other psychiatric disorders and symptoms such as bipolar disorder, pseudobulbar affect, euphoria sclerotica, anxiety and personality changes are also reported to be overpresented in MS patients. Psychiatric disorders in MS lead to significant disruption in patients' family, work and social life, affecting patients' quality of life in general. Moreover, they are reported to be associated with poorer adherence to MS medication. The literature concerning bipolar disorder and affect disorders in MS is rather scarce. The purpose of this article is to provide a critical review on the latter subject. This review focuses upon the recent findings with regard to the epidemiology and the comorbidity rates of bipolar and affect disorders in MS, questions raised about the potential underlying mechanisms that could explain such a high comorbidity, diagnostic issues and the recent developments in the treatment of those psychiatric disorders in MS. Despite the fact that the higher prevalence of psychiatric disorders in MS is well established, such disorders still remain underdiagnosed and undertreated. A shift in the clinical suspicion towards the psychiatric morbidity in MS patients and the optimal treatment of those disorders is fundamental.