Microparticles and exosomes are small vesicular fragments of cell membrane which are released from activated and apoptotic cells. Microparticles (MPs) range in size from 0.5-1.5 μm, and exosomes are 0.5 μm and under. For the purposes of this article we will refer to both categories as microparticles. They differ from apoptotic bodies based on their smaller size, intact structure, and lack of degraded nuclear material. MPs have been shown to be released from a variety of cell types including platelets, endothelium, vascular smooth muscle cells, dendritic cells, and tumor cells. Jimenez and others have shown that based on the stimulus and cell type the MPs released are both quantitatively and phenotypically distinct. More recent data have shown the proteomics of MPs released from human umbilical vein endothelial cells differ dependent on whether they are stimulated with PAI or TNF-α.