Receptor tyrosine kinases (RTKs) are single-pass transmembrane receptors that possess intrinsic cytoplasmic enzymatic activity, catalyzing the transfer of the g-phosphate of ATP to tyrosine residues in protein substrates. RTKs are essential components of signal transduction pathways that aect cell proliferation, dierentiation, migration and metabolism. Included in this large protein family are the insulin receptor and the receptors for growth factors such as epidermal growth factor, ยฎbroblast growth factor and vascular endothelial growth factor. Receptor activation occurs through ligand binding, which facilitates receptor dimerization and autophosphorylation of speciยฎc tyrosine residues in the cytoplasmic portion. The phosphotyrosine residues either enhance receptor catalytic activity or provide docking sites for downstream signaling proteins. Over the past several years, structural studies employing X-ray crystallography have advanced our understanding of the molecular mechanisms by which RTKs recognize their ligands and are activated by dimerization and tyrosine autophosphorylation. This review will highlight the key results that have emerged from these structural studies.