Binding proteins for the insulin-like growth factors (IGF-I and IGF-II) are increasingly being recognized as modulators of IGF actions in both inhibitory and stimulatory ways. At least three distinct classes of binding protein are thought to exist, differing in their primary structures and binding characteristics, although all are able to bind both IGF-I and IGF-II. This review outlines the purification and characterization of the binding proteins that have been identified to date, and describes the regulation of their production and of their levels in the circulation. Current views on their potential biological roles are also discussed.