Notch signaling is an evolutionarily conserved pathway and plays key roles in embryonic vascular development and angiogenesis. Multiple components of the Notch pathway are expressed in vasculature, and mice deficient for a variety of these components display embryonic lethality with vascular remodeling defects. Alteration of Notch signaling in various endothelial cells generates profound effects on angiogenesis in vitro. New evidence shows that Notch signaling from tumor cells is able to activate endothelial cells and trigger tumor angiogenesis in vitro and in a xenograft mouse tumor model. Selective interruption of Notch signaling within tumors may provide an antiangiogenic strategy.