Upon stimulation or apoptosis, eukaryotic cells shed membrane vesicles of submicron size. These so-called microparticles (MPs) are detected and characterized based on the exposure of antigens characteristic of their respective parental cells and on the increased distribution of negatively charged phospholipids to the outer membrane layer. Among the various hypothesized functions of MPs in both health and disease, one of the most studied is their possible role in hemostasis and thrombosis. In this context, special attention is paid to tissue factor (TF) exposed on a variety of MPs. MPs may have outstanding functional because of their ability to display "active" TF due to an abundance of negatively charged phospholipids on their surface. The rapid accumulation of TF-bearing MPs (TF+MPs) in a developing thrombus as well as the increased numbers and thrombogenic activity of TF+MPs in prothrombotic disorders indicate an important role in the pathogenesis of thrombosis. Nevertheless, isolation, quantification and antigenic characterization of TF+MPs proved challenging and a lively scientific debate is ongoing with respect to a reliable method to determine the cellular source of MP in vivo. Standardization of preanalytical procedures and development of more sensitive technologies are needed to improve our current understanding of the role of circulating TF+MPs in thrombosis.