More than 25 years have passed since immunoglobulin A (IgA) nephropathy was introduced as a disease entity independent of glomerulonephritis. It has been known that more than 30% of cases have gone into end-stage renal failure within 20 years, indicating the presence of a chronic active group in this disease. Histologically this disesase is composed of at least three types of tissue damage: (i) minimal inflammation including deposition of IgA-containing substances with minor matricial increase; (ii) acute lesions characterized by matricial damage of glomerular basement membrane (membranolysis) and/or mesangial matrix (mesangiolysis) with inflammatory cell accumulation and/or intrinsic cell proliferation; and (iii) chronic lesions mainly composed of postinflammatory sclerosis. The progression is actually accelerated by the frequency of acute lesions, resulting in increased glomerular sclerosis foci. In such a situation, the histologic grading and staging (G-S) system is proposed, with the aim of having a more precise understanding of the disease process. The histological grade (G) is estimated by the extent of acute glomerular and tubulointerstitial lesions, and the stage (S) is evaluated by the increase of extracellular matrices of the glomeruli and interstitial fibrosis. The evaluation of G and S is expressed semiquantitatively for more helpful clinical use.