Blood vessels are composed of two types of interacting cells. Endothelial cells form the inner lining of the vessel wall, and perivascular cells referred to as pericytes, vascular smooth muscle cells or mural cells which envelop the surface of the vascular tube. They are also called Rouget cells after their discoverer, Charles Rouget. Electron-microscope analyses first revealed the morphological character of pericytes. In general, pericytes possess a cell body with a prominent nucleus and a small content of cytoplasm with several long processes embracing the abluminal endothelium wall. They are embedded within the basement membrane of microvessels, which is formed by pericytes and endothelial cells. Pericytes play an integral role in the maintenance of the blood–brain barrier as well as several other homeostatic and hemostatic functions of the brain. These cells are also a key component of the neurovascular unit, which includes endothelial cells, astrocytes, and neurons. Pericytes provide a variety of functions such as capillary blood flow regulation, clearance and phagocytosis of cellular debris, angiogenesis formation of new blood vessels and regulating blood–brain barrier permeability. Recently, pericytes have gained new attention as functional and critical involvement to tumor angiogenesis and progression. Therefore as potential new targets for antiangiogenic therapies. Pericytes are complex. Their ontogeny is not completely understood, and they perform various functions throughout the body. This review article describes the current knowledge about the nature of pericytes and their functions during blood vessel growth, vessel maintenance, and pathological angiogenesis.