Eph receptors (Ephs, after erythropoietin-producing human hepatocellular receptors) are a group of receptors that are activated in response to binding with Eph receptor-interacting proteins (Ephrins). Ephs form the largest known subfamily of receptor tyrosine kinases (RTKs). Both Eph receptors and their corresponding ephrin ligands are membrane-bound proteins that require direct cell-cell interactions for Eph receptor activation. Eph/ephrin signaling has been implicated in the regulation of a host of processes critical to embryonic development including axon guidance, formation of tissue boundaries, cell migration, and segmentation. Additionally, Eph/ephrin signaling has recently been identified to play a critical role in the maintenance of several processes during adulthood including long-term potentiation, angiogenesis, and stem cell differentiation and cancer.