An understanding of the basic mechanics of hemostasis is fundamental to understanding disorders of hemostasis and therapies designed to alter coagulation. Generally, coagulation is divided into fibrin formation, fibrinolysis, platelet function, and natural anticoagulants. The coagulation cascade is a series of enzymatic steps designed to amplify the insult of initial trauma into the formation of a fibrin plug. Recent research has revealed how fibrin formation occurs in vivo rather than how it occurs in the test tube. The in vivo pathway for the purposes of this book is called the “new pathway” of coagulation. However, the two most common laboratory tests for coagulation and many books are still based on the test tube models of coagulation. It is important to learn about the older models of coagulation in order to understand these two laboratory tests and much of the classic literature.