Endothelial cells are the primary physical barrier between blood and tissue in microvessels. The other capillary and post-capillary venule wall cell is the pericyte. The literature on the biology of endothelium is appreciable but less is known about pericytes. Pericytes are morphologically, biochemically, and physiologically heterogeneous. Some of pericyte functional characteristics observed in vivo and in vitro are that they: regulate endothelial proliferation and differentiation; contract in manners that either exacerbate or stem endothelial cell junctional inflammatory leakage; function as a progenitor cell; synthesize and secrete a wide variety of vasoactive autoregulating agonists; synthesize and release structural constituents of the basement membrane and extracellular matrix. Pericytes are also involved in specific microvascular diseases. This review focuses principally on nonmesangial pericytes and specific activities such as the posttranslational, short-term responses that affect microvascular perfusion and permeability, and on pericyte-endothelial cell interaction.