Lupus nephritis is the archetype of diseases caused by the deposition of immune complexes. The characteristics which render only a portion of circulating immune complexes nephritogenic are not well delineated, but cat-ionic charge of antigen and/or antibody may contribute. Lupus nephritis is characterized by extreme diversity of clinical manifestations and pathologic features. Scrupulous monitoring of urinary sediment and renal function tests is necessary to identify renal involvement in a phase which is amenable to therapeutic intervention. Evaluation of renal biopsies also assists in development of indications for therapy. The pathology of lupus nephritis is classified primarily as mesangial, focal proliferative, diffuse proliferative, and membranous nephropathy. Indexes of activity and of chronicity provide a succinet description of the balance of reversible and irreversible disease. The activity index incorporates glomerular hypercellularity, karyorrhexis/fibrinoid necrosis, leucocyte exudation, hyaline thrombi, cellular crescents, and interstitial inflammation. The chronicity index incorporates glomerular sclerosis, fibrous crescents, tubular atrophy, and interstitial fibrosis. Delineation of the activity and chronicity indexes also facilitates the assessment of prognosis and the ordering of indications for therapy.