Blood coagulation appears to play an important role in the occurrence of cancer and its effects may be twofold. First, in patients with cancer, blood coagulation is activated in the direction of a prothrombotic state. Second, a procoagulant environment may promote cancer in different ways. In this chapter we discuss some of the mechanisms that may be involved in this interplay between coagulation and cancer. Blood coagulation proteins interact with cells in the vasculature to maintain hemostasis. However, many proteins that are involved in coagulation and anticoagulation, as well as fibrinolysis, are also found in extravascular tissues. In different organs, these proteins may be involved in cell-signaling mechanisms, through interaction with cell receptors like protease-activated receptors (PARs). Such interactions may drive inflammation, angiogenesis and cell proliferation. The potential procarcinogenic actions of proteases like thrombin may be counteracted by the anticoagulant and anti-inflammatory actions of the protein C-thrombomodulin mechanism. In the blood of cancer patients, the balance is usually shifted towards a procoagulant direction. The resulting excess thrombin- and fibrin-forming activity promotes venous thrombosis and may in the extravascular compartment stimulate cancer progression. The activation of platelets and their interaction with leukocytes may propagate this process. In addition to the therapeutic modulation of the prothrombotic environment, the induction of specific anticoagulant proteins including thrombomodulin may have effects on tumor growth or dissemination, but the nature of these effects still remains hard to predict. The interplay between cancer and blood coagulation merits further experimental and clinical research.