During the years that have intervened since the third volume of the International Life Sciences Institute (ILSI) Monographs on Pathology of Laboratory Animals, Digestive System, was published, new information of interest to pathologists has developed at a rather remarkable pace. Standardized nomenclature has been proposed and gained significant acceptance during the period since the first edition and is being utilized on an international basis. This has resulted in improvement in communi­ cation of pathologic data to regulatory agencies and in scientific publica­ tions worldwide. This monograph series and others sponsored by ILSI have had significant effects on improved communications and the inter­ national acceptance of standardized nomenclature. In this second edition, new formats have been used in some instances where more appropriate for the subjects to be covered. The format introduced in the first edition remains useful as it emphasizes the neces­ sity to recognize the morphologic features of pathologic lesions to iden­ tify them precisely. This identification is an essential first step toward development of new insights into pathogenetic mechanisms and their use in decisions eventually applicable to public health.