An enormous amount of research effort has been directed toward elucidating the mechanism by which substances are extruded from cells; and reviews have been written and symposia held in order to systematize the plethora of evidence made available. However, the approaches employed to study the secretory process have been so diverse that it is difficult, if not impossible, for one individual or even a group of individuals to keep abreast of all aspects of the field and to analyze them critically. Thus I undertook the writing of this volume with a great deal of trepidation. In searching for some starting point, I naturally considered as my primary focus the role of calcium in the secretory process, which has occupied my research interests for the past 13 years. But since so much experimentation has been carried out on this and related topics during the last decade or two, I felt it was still necessary to visualize this venture from two alternative approaches: (1) a more general one, which would cover the subject of calcium and the secretory process from a broad perspective, but of course not in great detail, and (2) a more specific one, restricting coverage to carefully defined limits but with comprehensive analysis of limited topics. The final course undertaken appears to lie somewhere between these two extremes.