The classification of lupus nephritis (LN) was first proposed by the WHO in 1974, first revised in 1980, then again in 1995, and revised for the third time in 2003 by the International Society of Nephrology and the Renal Pathology Society (ISN/RPS). The 2003 LN classification criteria divided LN into six types. Type I is a mildly lesioned LN of the thylakoid membranes: it appears essentially normal on light microscopy and immunocomplex deposits in the thylakoid region are visible on immunofluorescence. Type II is thylakoid proliferative LN: light microscopy shows thylakoid cell hyperplasia or increased thylakoid stroma with immune complex deposition in the thylakoid region, and immunofluorescence or electron microscopy shows a small amount of isolated subepithelial or subendothelial deposits. Type III is focal LN: less than 50% of all glomeruli are involved and may present as active or inactive lesions, focal, segmental or globular lesions, intra- or extra-capillary proliferative lesions. Type III(a) is an active lesion that presents as focal hyperplastic LN; Type III(a c) is an active lesion with chronic disease that presents as focal hyperplastic sclerosing nephritis; Type III(c) is a chronic lesion that presents as focal sclerosing nephritis. Type IV is diffuse LN: the affected glomeruli account for more than 50% of all glomeruli, and typical cases often have diffuse subendothelial immune complex deposits with or without thylakoid lesions. The glomerular lesions are classified as diffuse segmental LN (the affected glomeruli exhibit segmental lesions) and diffuse globular LN (the affected glomeruli exhibit globular lesions). Type IVs (a) is active segmental LN, IVg (a) is active spheroplastic LN, IVs (ac) is active with chronic segmental sclerosing LN, IVg (a c) is active with chronic spheroplastic LN, IVs (c) is chronic segmental sclerosing LN, IVg (c) chronic spheroplastic LN. Type V is membranous LN: light microscopy reveals subepithelial immune complex deposits with or without thylakoid lesions. Immunofluorescence or electron microscopy reveals spherical or segmental subepithelial continuous immune complex deposits, and type V is often coexisting with type III or IV. Type VI is progressive sclerosing LN: more than 90% of the glomeruli are spherically sclerotic. Active lesions are defined as the following: intracapillary cell proliferation, nuclear fragmentation, fibrinoid necrosis, glomerular basement membrane disruption, cellular or fibroblastic crescent formation, and platinum ears. Chronic lesions are defined as the presence of segmental or global glomerulosclerosis, fibrous adhesions, and fibrous crescent formation.