Various clinical syndromes illustrate the essential role of insulin in modulating somatic growth both in utero and after birth. The effect of insulin on growth is a consequence of direct effects transduced via its homologous receptor and post-receptor signaling pathways and indirect effects on other modulators of growth, such as the growth hormone-IGF axis. Recent insights into the post-receptor mechanisms of insulin signaling provide a scientific framework for the distinction between the traditional role of insulin as a major modulator of metabolism and its role as a promoter of growth.