The insulin and insulin-like growth factor family of peptides are proteins with similar structural characteristics whose functions have diverged during evolution. Recent studies have provided a great deal of information regarding structural characteristics of these peptides, the structures of the genes that control their biosynthesis, and determination of their receptor structures. Initially these factors were believed to function as classical hormones and the endocrine regulation of their blood concentrations was studied intensively. More recently several cell types have been shown to secrete these peptides and the autocrine and paracrine regulation of insulin-like growth factor secretion by cells and tissues has received extensive characterization. Following secretion into interstitial fluids the IGF's associate with high affinity, soluble binding proteins. These proteins are capable of modulating receptor binding and growth factor biological activity. The structure of these proteins and the variables that control their secretion are currently being determined. In spite of this information significant gaps in our knowledge exist regarding the mechanisms by which these peptides stimulate replication.