One of the most intriguing discoveries in molecular biology in the last decade is the existence of an evolutionary conserved and essential system, consisting of molecular chaperones and folding catalysts, which promotes the folding of the proteins in the cell. This text summarizes our current knowledge of the cellular roles, the regulation and the mechanism of action of this system. It has a broad scope, covering cell biological, genetic and biochemical aspects of protein folding in cells from bacteria to man. Particularly appropriate to researchers working in basic and applied aspects of molecular medicine, this volume should also prove useful as an up-to-date reference book and as a textbook for specialized university courses.