Optic neuritis is an inflammatory lesion of the optic nerve clinically associated with pain, decreased visual acuity, abnormal color vision, and afferent papillary defect. In acute cases, visual loss may progress rapidly in only a few days. The condition is bilateral in 30% of cases. While most incidents are idiopathic, about half of all patients presenting with optic neuritis will go on to develop multiple sclerosis with young adults having the greatest risk. Of all cases of MS, optic neuritis is the initial presenting symptom in 15-20%. Other less common diseases associated with optic neuritis include ischemia, sarcoid, systemic lupus erythematosus, syphilis, viral infection, toxoplasmosis, tuberculosis, and radiation therapy. MR is the best imaging modality for optic neuritis with thin slice axial and coronal images are preferred. T2-weighted images and short-tau inversion recovery (STIR) sequences show high signal intensity in the abnormal nerve with the nerve being frequently enlarged. Gadolinium is particularly useful, especially with fat saturation, with focal enhancement of the retrobulbar optic nerve sometimes being seen. Perivenous inflammation is responsible for the enhancement and can be seen in approximately 50% of patients with acute optic neuritis. Overall, MR sensitivity for optic neuritis is low, with much of the utility derived from its ability to exclude other causes of optic nerve compromise such as an orbital tumor.