In recent years because of their diagnostic importance techniques of immunohistochemistry have been increasingly employed; The use of specific cellular antibodies has allowed for the recognition of tumors of mesenchymal, epithelial, muscular, endothelial, and neuroendocrine origin, and, within these classes, for the revelation of different cellular populations. These techniques together with histochemical and ultramicroscopic studies help define not only the genesis but also the stage of differentiation and the cell function, which are important factors in the prognosis of tumors. These have proven to be particularly useful for the differential diagnosis of highly undifferentiated forms in which cellular morphology differs from that of the tissue of origin.