Insulin-like growth factor binding proteins (IGFBPs) belong to a family of at least six homologous proteins that bind insulin-like growth factors (IGFs) and modulate many of their biological actions. IGFBPs are produced by a wide variety of tissues and their circulating levels are regulated by both hormonal and metabolic factors. The binding activity of some IGFBPs is affected by phosphorylation, proteolysis, or association with extracellular matric (ECM) and plasma membrane. The IGFBPs may exert either inhibitory or potentiating effects on IGF actions. Recent evidence indicate that some IGFBPs are capable of direct actions on cells independent of the IGFs.