Our understanding of biological communication has grown significantly during the past decade. The advances in knowledge about the chemical nature of signals and their corresponding reception by specialized cells have led to identification, characterization, purification, cloning, and expression of specific receptor molecules. While the earlier literature emphasized com partmentalized treatment of informational molecules and their interaction with receptors, the progress in the recent past has allowed cross-fertilization in the examination of the of actions and mechanisms of steroid and protein hormones and other messengers. Investigators now have an increased ap preciation of the multiple effects of specific hormones and of the diverse responses by receptor proteins to closely related ligands. The task of compil ing this enormous literature into a focused treatise was undertaken with the launching of the series Hormones in Health and Disease. This latest volume, An Introduction to Cellular Signal Transduction, complements the previous monographs in the series and brings to the fore recent developments in the field of biochemical communication. This volume combines discussions on the basic tenets of the signal transduction process and its relevance to health and disease. While various chapters provide exhaustive dissection of specific topics for researchers in the field, the book is also an excellent vehicle for introducing students and new investigators to the subject. The contributors of the chapters are active and accomplished scientists brought together on a common platform by the editor, Dr.