Progress made in the last 20 years clearly indicates that the cell surface is an extremely dynamic structure involved in fundamental processes such as cell motility, innervation, and cell adhesion. Of particular interest is the finding that, in several tissues, the cell surface is differen­ tiated at the intercellular region, thereby providing communicating chan­ nels between apposing cells. Although our actual knowledge of the precise structures and mecha­ in the complex process of intercellular communication is still nisms involved scanty, evidence has been presented that ions and molecules diffuse from cell to cell, establishing a physiological continuum. Embryonic differentia­ tion, cell growth, neoplasia, electrical synchronization in nerve and mus­ cles, as well as immune response seem to be related to ceIl communication. In organizing this volume, it has been our intention to provide the reader with an actual review of the processes involved in intercellular communication in normal tissues as well as in neoplasia. We sincerely believe that the opinions and experiences described herein will be of help in establishing new perspectives for the future of this exciting new field of cell biology. We wish to thank all the colleagues who joined us in the organization of this volume, as well as Plenum Publishing Corporation for making its appearance possible.