Despite being one of the most relevant quantities to characterize the flow close to a solid boundary, direct and accurate measurements of wall-shear stress have not been available until recently. Extraordinary development of equipment and post-processing techniques over the past two decades have made possible to measure this quantity accurately. The mean shear stress at the wall T w is defined for Newtonian fluids as 12.1 t w = µ ∂ U ∂ y | y = 0, where µ is the fluid dynamic viscosity y is the wall-normal coordinate U is the streamwise mean velocity 394It is therefore a measure of the tangential force exerted by the incoming flow on the wall and by integrating over the surface it is possible to determine its impact on the aerodynamic actions on a submerged body. Its interest in the aeronautic industry stems from the fact that the integrated value of t w is the viscous component of the total drag. In commercial airplanes, viscous drag may contribute with as much as 50% of the total drag, while this fraction increases up to 90% in the case of submarines, thereby highlighting the importance of accurately measuring this quantity [1]. In addition to this, wall-shear stress (also known as skin friction) plays an important role in fundamental research in the fields of aerodynamics and fluid mechanics. Turbulence quantities and most prominently mean velocity profiles are commonly scaled with the so-called friction velocity u T = T w / p(p being the fluid density), and therefore small errors in the determination of the skin friction may lead to wrong conclusions regarding the functional form and asymptotic behavior of the velocity profile at very high Reynolds numbers. It is then possible to draw inaccurate conclusions about the nature of wall-bounded turbulent flows based on unreliable measurements of wall shear. An interesting example of this is described in the “The Clauser chart” section, where the popular Clauser chart method is described. This technique, which is based on an assumed form of the velocity profile in the so-called overlap region, was widely used for several decades in the wall-turbulence community. In fact, the results obtained with this technique were in some cases used to prove the validity of the initial assumptions [2]. Another case where accurate measurements of skin friction are extremely relevant is complex flows, such as pressure-gradient boundary layers or highly 3D configurations. Future improvements of the Reynolds-averaged Navier-Stokes (RANS) models used in industry to predict complex flows rely on an accurate characterization of these effects.