Tie-2, also known as Tek, is a transmembrane receptor tyrosine kinase (RTK) that functions as a receptor for Angiopoietin family proteins. It is expressed by embryonic and adult endothelial cells, hematopoietic stem cells, and a circulating population of proangiogenic monocytes. Tie-2 signaling is activated by Angiopoietins-1 and -4, while it can be either activated or inhibited by Angiopoietins-2 and -3. Tie-2 plays an important role in maintaining vascular integrity by mediating endothelial cell-smooth muscle cell communication and inhibiting endothelial cell apoptosis. It is also required for embryonic development of the endocardium. In addition, Tie-2 signaling mediates the quiescence of bone marrow stem cells in response to osteoblast-produced Angiopoietin-1. This quiescence is critical for maintaining an ongoing hematopoietic capability in the bone marrow.