The purpose of these lectures is to present general methods to construct boundary layers both in linear and nonlinear contexts. We will focus on semilinear cases (and therefore we will not address the Prandtl equation in these lectures). We will explain how the boundary layer sizes and profiles can be predicted in linear cases, together with some decay estimates. We will illustrate this method with several explicit examples: Ekman layers, reflection of internal waves in a stratified fluid… We will also tackle some semilinear problems, adding for instance a convection term to the previous examples.