The development of monoclonal antibodies to human tumor associated antigens has greatly facilitated the application of immunohistochemical techniques to analyze surgically removed tissues. During the last few years this approach has been utilized by a progressively increasing number of investigators to analyze malignant cells. Although monoclonal antibodies to tumor associated antigens have not become yet routine reagents in immunopathology, they have provided new information which could not be obtained with conventional antisera or histochemical procedures. The following are representative examples. TUmor associated antigens have been identified which display a restricted distribution in normal tissues and therefore may represent useful markers for radio imaging and appropriate targets for immunotherapy. In spite of undetec table differences with conventional histopathological approaches hetero geneity has been found in the antigenic profile of tumor cells within a lesion, in autologous lesions removed from different anatomic sites from a given patient and in lesions removed from different patients. Phenotypes of tumor cells have been identified which correlate with the biology of tumor cells and with the clinical course of the disease. From a practical view point the use of monoclonal antibodies in immunopathology has enhanced interactions between pathologists and immunologists, as exemplified by the present book. Such interactions have contributed to the application of basic research to clinical problems. The chapter of this book discuss investigations performed with monoclonal antibodies to antigens expressed by various types of normal and malignant human cells.