Receptor tyrosine kinases are cell-surface receptors that respond to numerous hormones and growth factors, including insulin, insulin-like growth factors, epidermal growth factor, and nerve growth factor. They activate highly conserved intracellular signaling pathways that regulate cell proliferation, differentiation, and metabolism, playing essential roles in developing and adult animals. This book examines the nature of these receptors and their ligands, the molecular mechanisms that they regulate within cells, and the roles of the receptors in normal physiology and control of embryogenesis. It also discusses how dysfunction of these mechanisms can contribute to cancer and other diseases.