This contribution seeks to illustrate the immunopathogenetic complexities of the syndrome of rapidly progressive glomerulonephritis. As with most of the clinical syndromes of glomerular disease, rapidly progressive glomerulonephritis may be dissected into a number of categories based upon combinations of clinical, morphologic and immunologic observations. It has been demonstrated that such observations have an important bearing on the natural history, prognosis and treatment of this syndrome. Much remains to be learned, particularly with respect to etiological factors, the genesis of extracapillary proliferation and the means of modifying or preventing these diseases.