This chapter will review the hematologic system and the various medications that an anesthesiologist will encounter. Heparin and warfarin are the most common anticoagulants encountered; however, more recently, new oral and intravenous anticoagulants targeting other factors of the coagulation cascade are being used in their place. As these new medications are developed, identification and knowledge of their characteristics will help guide patient care. Many of these new medications offer particular advantages as compared with the older medications and have been proven to be more effective when treating certain diseases. However, along with these advantages are disadvantages, particularly regarding bleeding and lack of effective reversal agents. Just as there have been advances in anticoagulation, newer antiplatelet agents have also been discovered, introducing a new set of advantages and disadvantages. Although transfusion medicine is discussed in later chapters, we will discuss potential alternatives to blood transfusions as well. Finally, this chapter will also briefly discuss various immunosuppressive agents that one may encounter in the OR setting.