Although the fact that blood clotted when it was released from the body was well known to Hippocrates, Plato, Aristotle, Celsus, and Galen, it was not believed to have any physiologic or pathologic significance. Petit, a surgeon, recognized in the 1730s that clotting was hemostatically important in amputations. Finally, the mechanism of clotting began to be studied by Buchanan (1838), who recognized thrombin; Hammarsten (1875), who purified fibrinogen; and Arthus (1890), who discovered the need for calcium. The fact that platelets existed and had a hemostatic function was developed in the 1800s. Not until the late 1940s did the explosion in the discovery of new clotting factors begin; they now number up to Factor XIII, plus many more that have no Roman numeral designation. Discovery of clotting factors led to their assays. The use of whole blood clotting times was improved by measuring the clotting times of plasma. This was followed by the partial thromboplastin time (PTT) and the activated partial thromboplastin time (APTT). The prothrombin time became an important laboratory test. Specific factors were assayed by progressively more specific tests. These included elements of the fibrinolytic and inhibitory systems. Platelets were counted and their functions measured by bleeding times, clot retraction, adhesion, and aggregation. The diagnosis of hemostatic disorders has improved in parallel with the discoveries of new factors and the development of their precise assays.