Gravitational lensing by black holes and their alternatives

Black holes have the ability to generate infinite number of images of any given source. These relativistic images are formed by light rays winding around the black hole several times. The phenomenology associated with these images is very rich, since these features are very sensitive to the metric structure of the black hole. Here, we review some aspects of gravitational lensing by black holes and consider some fundamental aspects related to alternative solutions, which do not reproduce Schwarzschild in the asymptotic limit and are supported by exotic matter.