Tie1 and Tie2 are receptor tyrosine kinase proteins that include a transmembrane domain. Tie1 and Tie2 are present on endothelial cells. This disclosure describes agents, such as antibodies, that bind to Tie1, Tie2, and Ang, including ones that inhibit endothelial cell activity and angiogenesis. The agents can be used to treat angiogenesis-associated disorders.