Boundary-layer theory is the cornerstone of our knowledge of the flow of air and other fluids of small viscosity under circumstances of interest in many applications. Thus many complex problems in aerodynamics have been clarified a study of the flow within the boundary layer and its effect on the general flow around the body. Such problems include the variations of minimum drag and maximum lift of airplane wings with Reynolds number, wind-tunnel turbulence, in other parameters. Even in those cases where a complete mathematical analysis at present impracticable, the boundary-layer concept has been extraordinarily fruit fill and useful.