Due to its structure, the glomerular capillary has a filtration function since it is interposed between the blood stream and the urinary space exposed to circulating plasma proteins, which are likely to form a deposit. The role of renal biopsy is to diagnose glomerulonephritis and systemic autoimmune diseases, on the basis of two samples: one for light microscopy and the other for immunofluorescence (IF). Electron microscopy has specific indications. IF is necessary to identify immune deposits. Renal biopsy has made possible the identification and characterization of lesions such as those related to the new viral diseases that emerged over the last decade: cryoglobulinemia and hepatitis C virus (HCV), the BK virus, the positivity of the C4d marker in humoural rejection of transplants, glomerular lesions due to monoclonal immunoglobulin. Besides, the recent molecular biology techniques as applied in renal transplantation are likely to improve the matching donors/recipients, and to treat graft rejections as revealed by gene expression in the renal tissue.