Anterior ischemic optic neuropathy (AION), one of the most prevalent and visually crippling diseases in the middle-aged and elderly, potentially bilateral, is due to acute ischemia of the optic nerve head. For a logical understanding of its pathogenesis, underlying causes, clinical features, and management, it is essential to comprehend the basic scientific issues involved; these are discussed briefly in this paper. Clinically, AION is of two types: (1) arteritic AION (due to giant cell arteritis) and (2) nonarteritic AION (due to other causes). Arteritic AION is an ophthalmic emergency because of its potential of causing rapid, bilateral complete blindness which is almost always preventable if treated immediately with large doses of systemic corticosteroids. Clinical parameters which help to differentiate the two type of AION, and their management are discussed.