The diagnosis of optic neuritis (ON) is made clinically based on typical signs and symptoms such as reduced vision and painful eye movements. Very often, ON is part of or associated with a systemic disease, in particular, multiple sclerosis (MS). Differential diagnosis is necessary in untypical cases with bilateral involvement, unusual age at manifestation or associated systemic and/or neurological symptoms. Neurological examination and cerebral MRI are standard in ON work-up. CSF analysis together with MRI scans allows MS risk assessment in ON patients. Further work-up is necessary in suspected systemic autoimmune disorders, sarcoidosis or infectious causes of ON such as borreliosis.