Thrombin is the main factor of mammalian blood coagulation, a complex system generating fibrin clots that physiologically prevents critical bleeding at local areas of tissue injury. There are many diseases, situations, or drugs that critically alter the blood matrix. Then, thrombin is massively generated in the systemic circulation. The individual reader of the present book will understand a bit more the complicated thrombin generating system - called coagulation. Understanding thrombin means understanding coagulation.