Microparticles are circulating, phospholipid rich, submicron particles released from the membranes of endothelial cells, platelets, leucocytes and erythrocytes. Investigation into their biological activity has revealed diverse actions in coagulation, cell signalling and cellular interactions. These actions are mediated through their phospholipid rich surfaces and the expression of cell surface molecules which reflect their cell of origin and its state of activation. Microparticle numbers are reported to be elevated in a number of conditions where vascular dysfunction and inflammation are important pathophysiological mechanisms, for example coronary artery disease or thrombotic microangiopathies. Currently, there are a variety of different methods used for the quantitation of circulating microparticles; however with standardisation their assessment may prove to be of clinical value, reflecting the state of the vasculature. Knowledge of the functional properties of microparticles will contribute to our understanding of the mechanisms underlying vascular dysfunction and prothrombotic states.