The insulin-like growth factors IGF-I and -II have effects on metabolism, growth, proliferation and differentiation in many tissues and cell types and are therefore important modulators of multiple aspects of physiology. IGF actions are largely mediated via the type 1 IGF receptor, though both peptides, particularly IGF-II, can bind to a second receptor known as the type 2 IGF/mannose-6-phosphate receptor. Access to these receptors is controlled by a family of six highly specific binding proteins (IGFBPs 1–6). Originally, the IGFBPs were thought of as IGF inhibitors, since their affinity for ligand is substantially higher than that of the receptors. More recently however, it has become apparent that modification of the binding proteins, for example proteolyis, phosphorylation or association with extracellular matrix, can influence IGFBP affinity for IGF and therefore IGF bioavailability and function.