Extracapillary crescent is an elementary lesion which can be superimposed to any type of glomerulonephritis, or constitute the main lesion as in vasculitis, or appear to be idiopathic. One must oppose cellular crescent which is partially reversible, to irreversible fibrous crescent. Cellular composition of crescents has been much controversial: initially considered as having an epithelial-cell origin, then an exclusive macrophagic origin, crescents are in fact composed of epithelial cells, monocytes-macrophages and lymphocytes all together. The former are prevailing when Bowman's capsule (BC) is intact, whereas macrophages predominate when there is extensive damage of GBM and of BC. The initial event in crescent formation appear to be breaches in glomerular capillary wall, leading the deposition of fibrin within Bowman's space. The consequence is the proliferation of parietal epithelial cells and of activated macrophages followed by a production, in series, of cytokines and growth factors.