The study of blood coagulation has been for more than fifty years a sort of academic battlefield for the physiologists. There is something about the obvious yet mvsterious conversion of fluid blood into solid clot that has intrigued a curiously large number of workers, many of them of international repute. It is unfortunate, perhaps, that many of them have devised and defended their own theories and terminologies, because the result is a mass of literature that is confusing to subsequent workers. It would also not be unfair to say that, until quite recently, this formidable bulk of experiment, theory, and controversy has contributed very little of practical value to the clinician. A few years ago, in fact, it would have been difficult to find material for a discussion such as this. It was -known that abnormalities of the coagulation system might have disastrous consequences for the patient, whether the blood failed to clot when required to, or clotted spontaneously in the vessels. Certain factors concerned in these abnormalities might have been understood, but nothing practical could be done for their prevention. Now, however, this part of the picture has changed, and is still changing rapidly. Academic research is at last bearing the fruit of practical application, saving many lives by such advances as vitamin-K therapy and the use of anti-coagulants in the prevention and treatment of thrombosis.The function of blood coaguldtion.-Its most obvious function is the arrest of bleeding. Anyone. who has observed the blood clot plugging the socket from which a tooth has just been removed might reasonably suppose that it is this plug that has stopped the bleeding. Moreover, in those conditions in which clotting is -delayed or defective, the patient is liable to bleed persistently or even fatally from, or into, any part of the body in which the vessels have been damaged. It is easy, therefore, to suppose rather vaguely that the clotting of blood stops bleeding, and that if the blood fails to clot, bleeding will continue. One cannot, however, explain by this loose hypothesis why persistent bleeding should occur in other conditions such as the purpuras and telangiectasia in which there seems to be nothing wrong with the clotting mechanism. In such cases normal coagulation is incapable of arresting the flow of blood even from a needle puncture. It is probable, therefore, that coagulation is only a part, though an essential part, of a haemostatic system that requires the operation of another factor to be effective. I, personally, believe that it is the active contraction of the damaged vessels, including the capillaries, that is the essential first stage of htemostasis (Macfarlane, 1941).