A biomarker is “a defined characteristic that is measured as an indicator of normal biological processes, pathogenic processes, or responses to an exposure or intervention, including therapeutic interventions”. Biomarkers can include molecular, histologic, radiographic, or physiologic characteristics. Biomarkers in multiple sclerosis are crucial and could help in diagnosis, prognosis and monitoring of treatment. This chapter describes multiple sclerosis biomarkers used in clinical practice as well as relevant potential biomarkers.