In arterial blood flow, the wall shear stress expresses the force per unit area exerted by the wall on the fluid in a direction on the local tangent plane. There is substantial evidence that the wall shear stress induced by the pulsatile blood flow in the arterial system affects the atherogenic process. It is now widely accepted that the vessel segments that appear to be at the highest risk for development of atherosclerosis are those with low wall shear stress or oscillating wall shear stress. The purpose of this article is to define wall shear stress, to introduce relevant concepts of fluid mechanics to nonexperts, and to critically review the various methods that have been used for the assessment of wall shear stress in animal and human blood circulation, paying special attention to the case of coronary arteries.