Immunohistochemistry (IHC), the utilization of monoclonal and polyclonal antibodies for the detection of specific antigens in tissue sections, is an extraordinarily powerful tool in the armamentarium of the diagnostic surgical pathologist. IHC is an important application of monoclonal as well as polyclonal antibodies to determine the tissue distribution of an antigen of interest in health and disease. It is widely used for diagnosis of cancers because specific tumor antigens are expressed de novo or up-regulated in certain cancers. IHC plays an important role in pathology, particularly in the subspecialties of oncologic pathology, neuropathology, and hematopathology.