Tissue factor (TF) is the primary cellular initiator of blood coagulation. At sites of vascular injury, formation of a TF:FVIIa complex leads to the generation of FXa, thrombin and the deposition of fibrin to limit hemorrhage. In contrast to its beneficial role in hemostasis, TF initiates life-threatening intravascular thrombosis in sepsis, atherosclerosis and cancer. More recently, TF has been proposed to play a role in other biological processes, including tumor-associated angiogenesis, metastasis and inflammation. Indeed, gene targeting of TF resulted in embryonic lethality, which appeared to be due to a defect in the yolk sac vasculature.