1. Pericytes are cells of microvessels (arterioles, capillaries and venules) that wrap around endothelial cells. They are most abundant on venules and are common on capillaries. 2. The pericyte population is highly variable between different tissues and organs, probably in a manner reflecting postarteriolar hydrostatic pressures. Pericytes are more abundant in the distal legs and feet, again suggesting a hydrostatic pressure-driven mechanical role for pericytes as protectors of microvessel wall integrity. 3. Pericyte alteration or degeneration is linked directly with microangiopathy in diabetes, scleroderma, hypertension, dementias and, possibly, inappropriate calcification of blood vessels. 4. Pericytes are functionally codependent on endothelial cells. Each cell type influences each others' mitotic rate and probably phenotypic expression. 5. Pericytes are not randomly located around microvessels. Instead, they are located adjacent to or over endothelial cell junctions of venules and especially over gaps between endothelial cells during inflammation. Pericytes are emerging as essential components of the microvessel wall, with metabolic, signalling and mechanical roles to support the endothelial cell.