A hundred years after the first description, many aspects of pericytes remain to be examined. Mesenchymal in origin, pericytes form an incomplete envelopment around the endothelial cells and within the microvascular basement membrane of capillaries and postcapillary venules. Morphologically, they appear as long, slender, polymorphic cells, showing an elongated cell body, from which arise longitudinal and circumferential branches. Cell bodies and cytoplasmic processes of pericytes, as well as the endothelial cells, are enveloped by the same basal lamina, except for where they make direct contacts with each other. The pericyte/endothelial cell contacts are peg and socket, adhesion plaques and gap junctions, making up structural mechanisms for force transmission and a possible receptor system for cells, in which the pericyte and endothelial cells respond to secondary signals generated in the other cells. Electron microscopic studies have revealed an elaborate network of cytoplasmic filaments. Pericyte intermediate filament proteins show species and tissue differences, expressing vimentin or vimentin and desmin. The pericytes also express protein typical of contractile cells, i.e. smooth muscle-specific isoforms of actin and myosin, cyclic GMP-protein kinase and tropomyosin. A gradual transition is observed between pericytes and smooth muscle cells in both terminal arterioles and venules. Several general functions for the pericytes have been postulated: contractability; permeability regulator; integrity maintainer; endothelial cell growth modulator; and cell progenitor with considerable mesenchymal potential.