It has been over 40 years since the original report by Salmon and Daughaday demon strating that the ability of GH to stimulate sulfation of cartilage was mediated by a "sulfation factor. " In the ensuing decades, it has become apparent that this "sulfation factor activity" encompasses a complex system ofligands (IGFs), receptors, and carrier proteins that are, in tum, responsible for a wide array of cellular actions. The IGF system has been demonstrated to be critically involved in both intrauterine and postnatal growth, and to have important implications in cancer biology as well, owing to the ability of the IGFs to function in endocrine, paracrine, and autocrine modes and given the wide distri bution of IGFs in virtually every organ system. The contributions to The /GF System reflect the wide span of interest in the IGF system and its implications for normal and abnormal growth and metabolism. The chapters have been divided into four broad sections: I. Molecular biology of the IGF system; II. Bio logical actions of the IGFs; III. IGF physiology; and IV. Clinical aspects of the IGFs. We have made every effort to highlight the major contemporary themes in IGF biology, but as is inevitable in such a fast-moving field, perspectives will continue to change as new information is accumulated.