The IGFBPs are a family of homologous proteins that have co-evolved with the IGFs and that confer upon the IGF regulatory system both functional and tissue specificity. IGFBPs are not merely carrier proteins for IGFs, but hold a central position in IGF ligand-receptor interactions through influences on both the bioavailability and distribution of IGFs in the extracellular environment. In addition, IGFBPs appear to have intrinsic biological activity independent of IGFs. The current status of research on IGFBPs is reviewed herein. Following a brief introduction to the entire IGF/IGFBP system, separate sections for each of the six cloned mammalian IGFBPs, the most extensive for IGFBP3, cover selected topics that emphasize the dynamics of IGFBPs--that is, their regulation in cells, their functionally important post-translational modifications, and their interactions in the cellular microenvironment--and how these dynamics influence physiological function.