Giant cell arteritis (GCA), the most common form of systemic vasculitis in adults, preferentially involves large and medium-sized arteries in patients over the age of 50. The classic manifestations are headache, jaw claudication, polymyalgia rheumatica (PMR), and visual symptoms, but 40% of patients present with a wide range of occult manifestations. Early diagnosis and treatment with prednisone can prevent blindness, the most feared complication of GCA. The pathogenesis of GCA is T-cell dependent and antigen driven. Clinical subsets of GCA appear to result from variable cytokine expression. The risk of developing thoracic aortic aneurysm is increased more than 17-fold in patients with GCA. GCA can also involve large arteries, especially the subclavian and axillary arteries. Color Doppler ultrasound, magnetic resonance imaging, and positron-emission tomography scanning are providing insights into the extent and pathogenesis of the disease but have not replaced temporal artery biopsy as the gold standard for securing the diagnosis. Two recently completed double-blind, placebo-controlled trials concerning whether methotrexate plus prednisone is more effective than prednisone alone reached conflicting conclusions.