Cellular microparticles support coagulation by exposure of negatively charged phospholipids and sometimes tissue factor. They are involved in inflammatory processes, the transfer of membrane antigens and bioactive molecules, and in the modulation of endothelial functions. Patients with disturbances in membrane vesiculation, leading to decreased numbers of circulating microparticles, present clinically with an increased bleeding tendency. In contrast, elevated numbers of microparticles are found in a great variety of diseases involving blood-vessel damage and hypercoagulability. Microparticles are also a major component of human atherosclerotic plaques. In view of their functional properties, cellular microparticles may be a missing link between cellular and plasmatic processes underlying atherosclerotic blood-vessel damage.