In recent years, cytology has played an increasingly important role in the diagnosis of disease, particularly those of neoplastic origin. It is not unusual for cytologic specimens to be the only diagnostic samples available from patients with cancer. Many ancillary tests traditionally performed on histologic material are now being performed on cytologic specimens. One such technique, immunocytochemistry (ICC), has already proved to be important in diagnostic tumor pathology. This atlas examines the value of ICC as well as its limitations in contemporary cytology. Methods have been refined and high-quality reagents are now available. As a result, technical problems no longer present a major concern in this field. This volume will briefly address the technical aspects of ICC by providing practical advice for the users and analytical aspects of ICC, including correct selection of markers for specific differential diagnoses and incorporation of results in final cytologic interpretation; illustrate the utilization of ICC in the resolution of some common diagnostic problems in fine needle aspiration and body cavity fluid cytology. ICC of undifferentiated malignant neoplasms, carcinomas of unknown origin, and most importantly its utilization in specific differential diagnoses that are based on cytomorpholoy and patient’s clinical history is also discussed. This practical handbook will allow for quick reference in the selection and interpretation of markers in specific differential diagnostic in the daily practice of diagnostic cytology.