This chapter described the physiology of forming a blood clot, including review of the coagulation cascade and platelet activation. It examined the various medications for prevention and treatment of thromboembolism. Anticoagulant mediations were summarized, including both injectable and oral forms. Special attention was paid to client education on warfarin therapy and enoxaparin injection technique. Antiplatelet medications were also described. Finally, thrombolytics were discussed. Bleeding risk was central to the discussion of each of these classes of medications.