Properties of membrane microparticles (MPs), as well as methods for their study are reviewed. Microparticles are vesicular fragments of a plasma membrane, which are detached from the surface of cells upon their activation and/or damage. An increase in intracellular calcium and subsequent remodeling of membrane cytoskeleton and redistribution of membrane phospholipids are key events leading to the MPs formation. Transfer of biologically active substances (proteins, lipids, and nucleic acids) from “parental” cell to other cells of the organism is the main function of MPs. MPs also have coagulation activity, that is, they are able to accelerate blood clotting. Procoagulant properties of MPs are determined by the expression on their surface of negatively charged phospholipids (first of all phosphatidylserine), which serve as substrates for assembling of coagulation complexes, and by the presence of tissue factor in some of them, the primary inducer of coagulation reactions. Methods of MPs counting and sizing are analyzed in this review with the indications of their limitations, advantages, and disadvantages. The main attention is focused on flow cytometry, the method that is most widely used in studies of MPs. The data on the coagulation activity of MPs originating from the blood cells (platelets, leukocytes, and erythrocytes) and endothelial cells are surveyed. Tissue factor-containing MPs derived from monocytes and endothelial cells have the highest capability to accelerate blood clotting. Information on the content of MPs of different cellular origin in the blood of healthy subjects and patients with thrombotic, inflammatory, and other diseases is presented.