Insulin-like growth factor (IGF) action is influenced by the insulin-like growth factor binding proteins (IGFBPs). Since 1988 eight forms of IGFBPs have been found which differ in molecular weight, amino acid composition, distribution in biological fluids and influence upon IGF activity. An important biological property of the IGFBPs is their ability to increase the halflife of the IGFs in the blood. They are able to act as potentiators of IGFs activity on the cell proliferation. As IGFBPs bind to cell surfaces, they may act either to deliver the IGFs to those surfaces either for the activation of specific receptors or cell responses independently of receptor activation. Posttranslation modification such as phosphorylation, glycosylation and proteolysis of IGFBPs influence their affinity to IGFs. In addition, the IGFBPs may also act as inhibitors to block the activity of the IGFs by preventing cellsurface binding.