The insulin-like growth factor (IGF) family is organized in a complex regulatory network at the cellular and sub-cellular levels. In the human the IGF system has a key physiological role in the development of the organism and maintenance of normal cellular function during foetal and postnatal life. The IGF system consists of three ligands, IGF-I, IGF-II and Insulin; three cell membrane receptors, IGF-I receptor (IGF-IR), insulin receptor (IR), and IGF-II receptor (IGF-IIR); six high-affinity IGF binding proteins, IGFBP-1 through -6, their specific proteases (IGFBP proteases) and membrane receptors (IGFBP-R) (Fig. 1).