Somatomedins are peptide hormones with insulin-like structure and effects. Their synthesis is governed by growth hormone. Somatomedins stimulate growth of cartilage tissue and other tissues. Synthesis of somatomedins occurs in various tissues, predominantly in the liver. For clinical practice determinations of somatomedin C (= IGF I) levels in blood have become valuable, since they give insights into the growth hormone secretory state, being low in growth hormone deficiency and high in acromegaly. In other growth disorders somatomedin C measurements can give new insights into the underlying pathogenesis. The role of IGF II and other peptides with somatomedin-like characteristics is presently under investigation.