Approximately 10 years have elapsed since the first volume of the International Life Sciences Institute (ILSI) Monographs on Pathology of Laboratory Animals, Endocrine System was completed. New information of interest to pathologists has developed at a rather remarkable pace during the intervening years. Exceptional progress has been made in the routine identification of cell products in endo crine cells. A better understanding has developed of the mechanisms involved in cell metabolism, particularly involving toxins and car cinogens. Clear concepts have developed concerning the significance of some pathologic lesions in the endocrine system and their relation to human health and risk assessment. Standardized nomenclature has developed significantly during the 1O-year period since the first volume and is being utilized on an international basis. This has resulted in significant improvement in communication of pathologic data to regulatory agencies and in scientific publications worldwide. This monograph series and others sponsored by ILSI have produced a significant effect on improved communications and the international acceptance of standardized nomenclature. In this second edition, new formats have been used where more appropriate for the subjects to be covered. In many cases, the format used in the first edition still is useful. It is still necessary to recognize the morphologic features of pathologic lesions in order to identify them precisely, an essential step toward development of new insights into pathogenetic mechanisms and their use in decisions eventually applicable to public health.