Insulin-like growth factors (IGFs) are fundamental cell regulators with an evolutionary conserved role synchronising tissue growth, development and function according to metabolic conditions. Although structurally very similar to insulin, the IGFs act in a very different way as cell regulators. Whereas insulin is stored in a specific gland and released when needed, the IGFs are stored outside of cells with soluble binding proteins. A very complex system of six IGF binding proteins, each of which exists in various modified states and interacts with other proteins, provides a sophisticated system for conferring specificity to provide a finely tuned system for local regulation at the tissue level.