In this talk we focus on a class of singular perturbation problems arising in the study of the dynamics of geophysical flows. Given a so-called ``primitive'' system of equations, the goal is to derive reduced models, under suitable assumptions on the fluid and on the scaling regime. The presence of a Coriolis term. encoding the Earth rotation, in the primitive system is the key element of the problems under consideration. We will discuss several aspects which enter into play in this context: the difference between the compressible and incompressible fluid cases, the presence of multiple scales, the formation of the Ekman boundary layers.