Regulation of wall shear stress (WSS) in microvessels is an important local mechanism regulating microvascular resistance, thus tissue blood flow and at the same time aiming to optimize circulatory energy consumption. The pressure drop across a microvascular unit indicates the loss of circulatory energy that is primarily due to WSS. In arterioles, an increase in WSS results in a substantial, endothelium dependent dilation, whereas in venules, it elicits only a limited increase in diameter. In arterioles, only dilator mediators, such as nitric oxide (NO), prostaglandins (PGI2/PGE2) endothelium derived hyperpolarizing factor(s) EDHFs, are released, whereas in venules, in addition to these mediators, constrictor prostaglandins are also released. WSS is the function of wall shear rate (WSR) and blood viscosity (hematocrit and plasma viscosity), which parameters can be substantially different in arterioles and venules. Thus it is likely that regulation of WSS is achieved by different mechanisms in arterioles and venules. In the arterial side WSS is determined primarily by high WSR (high velocity/narrow diameter), whereas in the venular side WSS is determined primary by the hematocrit related viscosity. Accordingly, it seems that at the arteriolar side WSS is regulated primarily by substantial increases in diameter, which can be achieved, since arterioles have a substantial basal tone and because WSR is high, thus diameter changes have less impact on apparent viscosity. In the venular side however, WSR is low, thus changes in diameter can substantially affect hematocrit-induced apparent viscosity of blood. Thus in the microcirculation there is a complex interrelationship between rheological parameters, structural and functional properties of microvascular network. The nature of endothelial mediation of WSS seems to be gender specific, and it can change with age and in diseased conditions. The primary sensors of changes in WSS are likely to be the glycocalyx, as part of endothelial surface layer and platelet endothelial cell adhesion molecule (PECAM), whereas the cytoskeleton and integrins are the next serially coupled molecules of mechanotransduction. There are still several controversial issues however, such as the role of increase in [Ca2+]i and reactive oxygen species in the release of various endothelial mediators and the nature and mediation of WSS-induced responses in cerebral microvessels. The fascinating role of endothelium in the mechanotransduction of shear stress into vasomotor response is still an open field for further discoveries.