Renal involvement is a common and often severe complication of anti-neutrophil cytoplasmic autoantibody (ANCA) associated vasculitides (AAV). With the exception of Churg–Strauss syndrome (CSS), where kidney involvement is not a prominent feature, renal disease is present in about 70% of patients with Wegener's granulomatosis, now called granulomatosis with polyangiitis (GPA) and in almost 100% of patients with microscopic polyangiitis (MPA). Kidney involvement is generally characterized by a pauci-immune necrotizing and crescentic glomerulonephritis with a very rapid decline of renal function (rapidly progressive glomerulonephritis). Even though there are not qualitative differences in glomerular lesions in patients with GPA or with MPA, chronic damage is significantly higher in MPA (and/or P-ANCA positive patients) than in GPA (and/or C-ANCA positive patients). If untreated necrotizing and crescentic glomerulonephritis has an unfavorable course leading in a few weeks or months to end stage renal disease. Serum creatinine at diagnosis, sclerotic lesions and the number of normal glomeruli at kidney biopsy are the best predictors of renal outcome. Corticosteroids and cyclophosphamide (with the addition of plasma exchange in the most severe cases) are the cornerstone of induction treatment of ANCA-associated renal vasculitis, followed by azathioprine for maintenance. Rituximab is as effective as cyclophosphamide in inducing remission in AAV and probably superior to cyclophosphamide in patients with severe flare, and could be preferred in younger patients in order to preserve fertility and in patients with serious relapses.