© 2018 Elsevier Inc. All rights reserved. Transfusion medicine physicians and laboratory scientists are confronted daily with hemostasis and thrombosis-related concerns as they select and administer blood components, coagulation factor concentrates, anticoagulants, and agents to manage anticoagulant therapy. This chapter provides an introduction and overview of hemostasis. Primary hemostasis focuses on platelet function and interactions with the vasculature, endothelium, and the coagulation mechanism. Secondary hemostasis focuses on the coagulation cascade and is subdivided into the extrinsic, intrinsic, and common enzymatic pathways. Coagulation also includes control systems such as protein C, protein S, and antithrombin, and fibrinolysis. Depending on the defect, hemostasis disorders may be congenital or acquired, resulting in hemorrhage or thrombosis. The questions in this chapter will explore normal hemostasis, disorders of hemostasis, and the laboratory assays that predict, identify, and monitor treatment of hemorrhage and thrombosis.