Anterior ischaemic optic neuropathy (AION), a common, visually crippling disorder, is discussed, with particular emphasis on differentiating AION due to giant cell arteritis (arteritic AION) from that not due to it (non-arteritic AION). Giant cell arteritis is an ophthalmic emergency because of imminent danger of bilateral total blindness, which is almost always preventable if the disease is quickly identified and treated urgently and aggressively. My studies have revealed that the best means of differentiating arteritic from non-arteritic AION is a combination of information from the following: systemic and visual symptoms of giant cell arteritis, high erythrocyte sedimentation rate and C-reactive protein, early massive visual loss, chalky-white optic disc swelling, associated cilio-retinal artery occlusion, massive non-filling of the choroid on fluorescein fundus angiography and temporal artery biopsy. Management of giant cell arteritis and of arteritic AION is discussed. Current misconceptions about AION are pointed out.