Although blood capillaries were first observed through a flea-lens microscope by Malpighi in 1661,200 more years elapsed before the cellular nature of the vessel wall was conclusively demonstrated. Beginning with the middle of the 19th cen­ tury, our knowledge of the histological organization of blood vessels has steadily increased. However, the endothelium, which for a long time was considered to be just an inert barrier lining, had been barely explored until three decades ago. Since then, there has been an upsurge of interest in the fine structure and function of endothelial cells. Intense in vivo and in vitro investigations have revealed that the endothelial cell is a key element in a wide variety of normal activities and diseases. A large number of investigators and laboratories have been attracted to endothelial cell research, thus supporting the expansion of the continuously grow­ ing and diversifying field of endotheliology. The number of articles published annually on this subject has increased from a few score at the beginning of the 1970s to more than a thousand in recent years, and an increasing number of journals, books, societies, and symposia focused primarily on the vascular en­ dothelium have marked the last decade.