Giant cell arteritis is a potentially systemic disease of medium-sized and large caliber arteries, showing a preferential manifestation in the extracranial branches of the carotid artery. The diagnosis is oriented to clinical and histomorphological criteria which will be critically reviewed. Particular emphasis is placed on the differentiation from normal aging processes and from healing stages under steroid therapy. In addition, the advances in our understanding of the disease pathomechanism during the last 10 years will be briefly presented as the basis for the hitherto empiric steroid treatment.