Microvessels are composed of two types of cells: endothelial cells and pericytes. Pericytes are elongated cells of mesodermal origin that partially surround the endothelial cells of small vessels. As pericytes contain contractile muscle filaments on their endothelial cell side, they have long been regarded as just microvascular counterparts of smooth muscle cells, thus being implicated in the regulation of capillary tone. However recent understanding of pericyte biology suggests that pericytes play an important role in the maintenance of microvascular homeostasis. Indeed, loss or dysfunction of pericytes has been considered to play an active part in the pathogenesis of various types of disorders. In this study, we review the biology of pericytes and the pathological role of pericyte loss or dysfunction in various devastating disorders such as diabetic retinopathy, atherosclerosis and tumor angiogenesis