  We analyse the necessary and sufficient conditions for the occurrence of
superradiance. Starting with a wave equation we examine the possibility of
superradiance in terms of an effective potential and boundary conditions. In
particular, we show that the existence of an ergoregion is not sufficient; an
appropriate boundary condition, e.g. only ingoing group velocity waves at an
event horizon, is also crucial. After applying our scheme to the standard
examples of superradiance, we show that analogue models of gravity without an
event horizon do not necessarily exhibit superradiance. Particularly, we show
that the superradiant phenomenon is absent in purely rotating inviscid fluids
with vorticity. We argue that there should be a catalogue of superradiant
systems that can be found by focusing on the necessary and sufficient
conditions outlined below.
