Microparticles (MP) are small membrane-bound vesicles that circulate in the peripheral blood and play active roles in thrombosis, inflammation and vascular reactivity. While MP can be released from nearly every cell type, most investigation has focused on MP of platelet, leucocyte and endothelial cell origin. Cells can release MP during activation or death. Flow cytometry is the usual method to quantify MP; the small size of these structures and lack of standardization in methodology complicate measurement. As MP contain surface and cytoplasmic contents of the parent cells and bear phosphatidylserine, antibodies to specific cell surface markers and annexin V can be used for identification. Through various mechanisms, MP participate in haemostasis and have procoagulant potential in disease. MP contribute to inflammation via their influence on cell-cell interactions and cytokine release, and MP also function in mediating vascular tone. In several disease states characterized by inflammation and vascular dysfunction, MP subpopulations are elevated, correlate with clinical events, and may have important roles in pathogenesis. In the rheumatic conditions such as rheumatoid arthritis and systemic lupus erythematosus, MP are potentially important markers of disease activity and have an increasingly recognized role in immunopathogenesis. It is clear that MP play an important role in atherosclerosis, and study of these structures may provide insight into the link between chronic inflammatory conditions and accelerated atherosclerosis. As biomarkers, MP allow access to usually inaccessible tissues such as the endothelium. Further research will hopefully lead to interventions targeting MP release and function.