I will review some results on lensing by black holes and other compact objects in the presence of a plasma. In particular, I will discuss the criteria of separability for the Hamilton-Jacobi equation for light rays and the resulting equatons for the photon sphere and for the shadow of stationarily rotating compact objects. I will also present formulas for the bending angle and criteria for multiple imaging. The relevance in view of actual observations (in the radio regime) will also be discussed.