In recent years great progress has been made concerning the reasons of many paradoxical data recorded in disordered arrest of bleeding. They are emphasized in the present paper where some of the still unsolved problems are merely mentioned. From the data recently disclosed, the role of platelets in hemostasis appears even more prominent than before : they are not only the building stones of the dam which stops the blood flowing from small arteries and veins, but they also bring with them in their “plasmatic atmosphere” the cement, i.e. the adsorbed blood clotting factors, which causes them to be firmly united into a sort of concrete. Then this biological concrete is transformed into a sort of armoured-concrete by means of fibrin strands originated in the free circulating plasma. These strands strengthen the hemostatic plug and fasten it more firmly to the lips of the wound. Analysis of hemorrhagic diseases discloses that besides platelet and/or blood clotting deficiencies, a peripheral factor not infrequently contributes, sometimes for a great part, to prolonged arrest of bleeding. Such analysis also confirms usual “multicausation” in disordered hemostasis.