This paper aims to give a detailed presentation of long-wave instabilities of shear layers for NavierStokes equations, and in particular to give a simple and easy-to-read presentation of the study of the OrrSommerfeld equation and to detail the analysis of its adjoint. Using these analyses, we prove the existence of long-wave instabilities in the cases of slowly rotating fuids, slightly compressible fuids, or fuids with Navier boundary conditions, under smallness conditions.