Microscopic polyangiitis, Wegener's granulomatosis, Churg-Strauss syndrome, and pauci-immune necrotizing glomerulonephritis share pathogenic, pathological, and clinical features. They all involve capillaries, venules, arterioles, and small arteries. Approximately 90% of patients have autoantibodies either to myeloperoxidase (MPO-ANCA) or to proteinase 3 (PR3-ANCA). The clinical manifestations of ANCA-small vessel vasculitis are protean. These can be limited to the kidney alone, or may involve the upper respiratory tract, the lungs, the skin, or a number of other organs in various combinations. The characteristic feature of the glomerular lesion is a focal necrotizing glomerulonephritis associated with crescent formation and little or no glomerular staining for immunoglobulin by immunofluorescence microscopy. The renal manifestations can present as a rapidly progressive glomerulonephritis or that of a more indolent, remitting, and relapsing course that leads to substantial glomerulosclerosis. The two main prognostic markers of the long-term outcome are the presence of pulmonary hemorrhage (which accounts for at least half of all deaths) and the entry serum creatinine. The higher the entry serum creatinine, the higher the risk of developing end-stage renal disease. The treatment of ANCA-small vessel vasculitis and glomerulonephritis rests primarily on the use of induction high-dose corticosteroids and cyclophosphamide. Patients with pulmonary hemorrhage also benefit from plasmapheresis. With the use of an alkylating agent, the rate of remission is of the order of 75%, but relapses occur in about 30% of patients who achieve a remission, and in about 17% of patients after renal transplantation. Despite the improved outcome of patients with ANCA vasculitis in the recent decade, their long-term prognosis continues to be primarily determined by a rapid diagnosis, and the prompt institution of therapy.