This chapter is devoted to the mechanism that prevents loss of blood (hemorrhage or bleeding) after a damaged blood vessel. The same processes are involved in the development of thrombosis. Both of these pathological events come under the heading of ‘Blood Coagulation’. This process is based on a series of interactions between proteins (called factors and identified with roman numerals), that lead to the formation of a mesh able to stop the loss of blood, the main constituent of which is Fibrin. We will describe one of the most important disorders of blood coagulation, not only because of its pathophysiological basis but also for its historical relevance.