The NATO Advanced Study Institute on "Cellular and Molecular Control of Direct Cell Interactions in Developing System" has been attended by 15 invited main lecturers and 60 participants. According to its purpose senior scientists, postdoctoral trainees and graduate students working in areas like biology, biochemistry, electrophysiology, medicine etc . . . could discuss their common interest in the various structural, ultrastructural, molecular and functional aspects of cell interactions in developing in vivo and in vitro systems. Whereas the topics of the first week have been mostly concerned with the general aspects of cell interactions in embryogenesis (section I and II of this book), the second week has been mainly devoted to the structures and functions of the direct cell contact sites at the membrane level as gap junctions, including electrophysiological aspects, dye coupling and selective cell-cooperation in some model systems as the neuro-muscular junctions (section III-V of this book). A multidisciplinary and stepwise approach, from initial cell contacts in early embryogenesis up to well defined selective cell cooperation, appeared to be an efficient means to provide answers to the question of how cells control, in a dynamic system as given in a differentiating embryo, their multiple temporary and permanent interactions so necessary for ordered cell positioning, cell linking and well established cell-to-cell communication.