Hemostasis commands special attention among physicians and investigators. The importance and appeal of this discipline rests on exciting scientific advances as well as its obvious clinical relevance. The mechanisms underlying the initiation and control of blood coagulation constitute a complex but beautifully integrated system of cellular and molecular interactions that fulfill a biological function of crucial importance. As summarized in Chapter 1, the circulation of blood cells and plasma through the vascular tree is essential for providing the body’s organs and tissues with nutrients and oxygen and for defense against infection and inflammation. Platelets, endothelial cells, and coagulation proteins cooperate in a complex and dynamic way to repair leaks in the vasculature and protect against hemorrhage in a high-pressure circulatory system.