  Superradiant scattering processes are studied in general relativistic systems
which, unlike rotating and/or charged black holes, do not exhibit an event
horizon. Inspired by Zel'dovich's seminal works on the amplification of waves
by a rotating cylinder, we analyse, in the context of General Relativity, the
possibility of superradiance for electromagnetic waves reflecting off a
rotating star and for charged scalar perturbations impinging on a charged
sphere. The role of energy dissipation in these systems is analysed and
compared with the role of the event horizon in black hole superradiance.
