THE insulin-like growth factors (IGFs) and their receptors and binding proteins constitute a family of cellular modulators that play essential roles in the regulation of growth and development. The IGF ligands include three structurally related peptides: insulin, IGF-I, and IGF-II. Unlike insulin, IGF-I and IGF-II are expressed ubiquitously, albeit in a highly regulated manner (see reviews in Refs. 1-5). The biological functions of the IGFs are mediated by a family of transmembrane receptors, which includes the insulin, IGF-I, and IGF-II/mannose-6-phosphate (M-6-P) receptors. While the IGF-I receptor is the primary mediator of IGF action, the insulin and IGF-II/M-6-P receptors may also mediate some of these functions (Fig. 1) (6, 7). The biological actions of the IGFs are modulated by a family of at least six IGF-binding proteins (IGFBPs) that are found in the circulation and in extracellular compartments and are produced by most tissues. The IGFBPs are capable of inhibiting or enhancing IGF effects and may even have ligand-independent effects (see reviews in Refs. 8–11). This review will focus on the IGF-I receptor and will discuss various aspects of its molecular structure and the cellular effects elicited by its activation.