Although many tumors may be diagnosed by light microscopy alone, a few require further investigation. Selective use of antibodies by immunohistochemical techniques solves most diagnostic problems. The explosive discovery of many new antibodies often raises more questions than it answers, and electron microscopy still plays a significant role when used judiciously. It is important to recognize that every new antibody must be adequately assessed in the clinical situation and that its specificity, or lack thereof, is appreciated by both clinician and pathologist. With every new antibody, we increase our knowledge of basic disease processes. In the future, antibodies will be part of the pathologist's armamentarium for predicting disease outcomes. Not only will the cell of origin be identified but proliferation rates and production of various factors that influence metastatic potential will be delineated.