Optic neuritis (ON) refers to any inflammatory disorder of the optic nerve. In clinical practice ON is mainly diagnosed by ophthalmologists and less frequently by neurologists. ON diagnostic criteria are included in different classification systems both in neurologic and ophthalmologic fields. Diagnosis of ON is still very unsatisfactory. Indeed diagnostic criteria are not uniform and therefore the diagnosis is still mainly formulated according to the clinical experience only. A consensus on practice guidelines for ON diagnosis might be useful. Ocular pain is a milestone in ON diagnosis, but it is too often mistreated by both the patient and the clinician. The International Headache Society (IHS) Classification of Headache Disorders provides in its 1988 and 2004 versions the diagnostic criteria for ON. These criteria are not spread and followed by the large majority of neurologists, but they are mainly applied by the experts in headache disorders. On the other hand, ON is a disorder widely encountered by neurologists and ophthalmologists. The latest IHS version defines the criteria of the pain features more precisely, but it is still unsatisfactory. In a future revision, the pain should be further detailed. Further studies aimed at validation of the diagnostic criteria of ON are strongly needed.