Since 1952, postgraduate courses for practising physicians and speci­ alists have been given by the Medical Faculty of the University of Leiden in the Boerhaave Quarter, in which most of its clinics and laboratories are located. During these years, recent advances in a wide variety of m~dical fields and subjects have been discussed by distin­ guished speakers from many countries. The steadily increasing atten­ dance has shown that, as could be expected from the rapid progress of modern medicine, there is a widely felt need for this form of postgra­ duate study. In 1957, therefore, the Leiden Medical Faculty appointed a permanent committee for the organization of postgraduate medical education. Of the courses given since then, certain material proved to have sufficient immediate scientific value to justify publication, and it now gives the Committee great pleasure to announce that in collaboration with the Leiden University Press it will publish the Boerhaave Series for Postgraduate Medical Education. The first volume of this new series is the product of the course on Human Blood Coagulation given in Novem­ ber 1968. It is our hope that this book will prove valuable not only to those who participated in the course but also to many others working in this and associated fields.