Grau. teurer Freund. ist aUe Theorie Und griln des Lebens goldner Baum All theory. dear friend. is gray But the golden tree of actual life springs ever green -Goethe Progress achieved in the last 25 years indicates that the establishment of inter­ cellular junctions at the area of cell contact represents an important mechanism of intercellular communication. Evidence is available that intercellular channels are essential for electrical synchronization in excitable tissues and for the diffusion of molecules from cell to cell. This process of cell-to-cell communication is so reliable that it was pre­ served throughout the evolutionary scale. As evolution generates diversity it is not surprising that gap junctions are not the same in all systems. It is known, for 2 instance, that junctional permeability is reduced by high free [Ca +]j in some cells or by a fall in pHi in others, or enhanced by cAMP. Our knowledge of the physiological modulators of junctional permeability is still scanty. Moreover, the compartmentalization of the intracellular milieu represents an additional complication in the interpretation of many studies of cell-to-cell communication. The present volume represents an effort to provide the reader with an actualized view of the mechanisms of cell communication and of the physiologi­ cal and pathological implications of junctional and nonjunctional communica­ tion. Let us hope that the content of this book helps future studies in establishing a better picture of the cellular and molecular mechanisms involved in the process of intercellular communication.