When established four years ago, the scope of this international series in electron microscopy essentially was to provide an opportunity for the pUblication of selected review contributions by specialists in ultrastructural research. Previous volumes presented over the last three years have focused on special topics of present interest in ~'ontemporary biomedicine such as endocrine cells, reproduction, and connective tissues. In these fielCls, in fact, integrated methods of electron microscopy have contributed much to generate new ideas and concepts of general value in both basic and clinical applications. The Ultrastructure of the Digestive Tract basically follows the same guidelines and style of the other books in the series and is an invited collection of selected contributions of authors from various laboratories active in the field of electron microscopy. Therefore, although the various chapters consist of individual topics, they nevertheless should be considered as interrelated contributions of specific subjects in the field. The idea was to have critical reviews of aspects previously published elsewhere by experts in the field who, as a rule, include other relevant information in their articles in order to update and enrich the subject. This book contains fifteen chapters by renowned electron microscopists. Each chapter, according to the policy of the editors, reviews a particular topic in great detail, providing updated information, study methods and results, authors' ideas on future investigative approaches, and possible guidelines for forthcoming work. We hope that this book will be useful to cell biologists, morphologists, physiologists, and pathologists.