Theoretical modeling of perturbations introduced into turbulent boundary layers by an isolated hump can be carried out within the scope of the Reynolds-averaged Navier-Stokes equations. The main obstacle is the closure problem. Having chosen a closure model that is suitable for the boundary layer, one may expect that it will not be adequate for the perturbations. As is known from asymptotic analysis of laminar boundary layers with roughness elements, the physics and the governing equations depend on the roughness length, width, and height. Even at small heights of roughness element, one should expect that the governing equations are nonlinear. These and other questions relevant to roughness-induced perturbations are discussed in the lecture.