Over the past ten years, several growth factor receptors have been shown to be ligand-regulated tyrosine kinases. Tyrosine kinase activity is essential for signal transmission, suggesting that phosphorylation cascades may play an important role. Considerable effort has gone into understanding the structure and function of tyrosine kinase receptors in order to define their mechanisms of signal transmission. However, the protein substrates of the receptor kinases have proven to be difficult to isolate and clone. This review focuses on the receptors for insulin, epidermal growth factor, and platelet-derived growth factor. They are all tyrosine kinases, but emerging evidence suggests that they utilize multiple separate signal transduction pathways. Work carried out during the next several years should yield considerable insight into the complexity of the components which interact with these tyrosine kinase receptors to regulate cellular growth and metabolism.