Blood flow influences the luminal side of vessel wall by a frictional force known as wall shear stress (WSS). This local hemodynamic factor is an important determinant of endothelial gene expression and of endothelial phenotype. The effect of WSS depends on its magnitude and direction, as well as on the characteristic of vessel geometry and blood flow. WSS seems to be a critical determinant of vessel caliber, vascular remodeling and restenosis and plays a role in intimomedial hyperplasia. Research is concentrated especially on the influence of WSS on atherosclerotic plaque formation.