Giant cell arteritis (GCA) is a form a large- to medium-vessel vasculitis that involves the aorta and its branches. Polymyalgia rheumatica (PMR), a syndrome of muscle pain and stiffness in the neck, shoulders, and hips, often occurs with GCA but can occur independently. GCA predominantly affects the second- to fifth-order aortic branches, often in the extracranial arteries of the head. The mean age at diagnosis of GCA is approximately 72. The diagnosis of GCA is made usually by biopsy of the temporal artery. GCA associated with granulomatous inflammation within the blood vessel wall, but granulomatous changes are not always evident early in the disease. Giant cells occur in a slight majority of cases of positive temporal artery biopsies. Estimates of the frequency of giant cells in temporal artery biopsy tissues are on the order of 50–80%. In GCA, clinical symptoms of local vascular inflam-mation and vascular insufficiency are usually accompanied or preceded by a systemic inflammatory process. Visual loss is the most feared complication of GCA. Visual loss may occur through the syndrome of anterior ischemic optic neuropathy, caused by narrowing of the posterior ciliary artery, as well as through other arterial insufficiency syndromes. Glucocorticoids are the cornerstone of treatment for GCA and PMR. PMR occurring in the absence of GCA requires a lower dose of prednisone for disease control.