Insulin-like growth factors (IGF-I, IGF-II) are important regulators of cell division and differentiation. In their free form, IGFs form a complex with specific binding proteins (IGFBPs), six of which have now been characterized. These IGFBPs differ in their ability to bind IGF-I and/or IGF-II, and, depending on their location and metabolic circumstances, they inhibit or augment IGF action to varying extents. New findings have changed our understanding of IGFBPs, which are now considered to be major regulators of IGF action and IGF transport proteins between compartments. Recently, the IGF-inde-pendent action of IGFBPs was discovered. Levels of IGFBP-3, for instance, are important indicators of states of altered growth hormone secretion and action, as well as being important during treatment with growth hormone or IGF-I. IGFBP-1 and -2 levels reflect changes related to nutrition, insulin secretion, foetal development and malignant state. The unravelling of the intricate interactions of IGFs, IGFBPs and their targets is bound to influence our understanding of the physiology and development of biological systems. Likewise, the possibility of regulating the IGFBP system is likely to open up new fields in the treatment of diseases in the future.