The majority of patients with Multiple Sclerosis (MS) experience fatigue and for many susabjects concerned it is the most disabling symptom. Fatigue is most prominent in the afternoon and may be aggravated by heat. It has a tremendous negative impact on quality of life and is often one of the major reasons for early retirement and unemployment. Against further assumptions, fatigue can occur at all stages and is often present at the onset of the disease. Reliable assessment however, is difficult as it is a subjectively perceived lack of physical and/or mental energy interfering with intended activities and has to be differentiated from depression, consequences of sleep disorders, cognitive decline, and side-effects of medication. Moreover, fatigue is not directly related to overall disease evolution, to disability levels or localized lesions, although an association with dysfunction of fronto-thalamo-basal-ganglia circuits seems likely. Several therapeutic approaches including pharmacological as well as non-pharmacological strategies are available but an evidence-based specific gold-standard for the treatment of fatigue is still missing.