During the past few years remarkable progress has been achieved in the understanding of the pathogenic mechanisms leading to vascular inflammation and injury in giant cell (temporal) arteritis. T lymphocytes are activated by specific recognition of a putative antigen residing in the arterial wall and, subsequently, activate macrophages that undergo a functional differentiation and contribute to vessel damage through various pathways. Vascular response to inflammation amplifies the inflammatory response through neovascularization and adhesion molecule expression. We are beginning to appreciate that products released by infiltrating inflammatory cells may play an important role in vessel occlusion and resulting ischemic complications. Concomitantly, synovitis underlying polymyalgia rheumatica musculoskeletal symptoms has been immunopathologically characterized and the nature of its relationship to giant cell arteritis is discussed. Although some components of the disease are highly corticosteroid responsive, other underlying pathogenic mechanisms may remain active. Long-term outcome is heterogeneous in patients with giant cell arteritis. Much less is known about the pathogenesis of Takayasu's arteritis. Recent work supports its association with HLA class I antigens, which may differ in different geographic areas or ethnic groups. Because Takayasu's disease expression may vary in different ethnic settings, this possibility has led to the proposal of new diagnostic criteria. Finally, the role of new imaging techniques in diagnosis and assessment of disease activity is discussed.