  We review some results on the connection among supergravity central charges,
BPS states and Bekenstein-Hawking entropy. In particular, N=2 supergravity in
four dimensions is studied in detail. For higher N supergravities we just give
an account of the general theory specializing the discussion to the N=8 case
when one half of supersymmetry is preserved. We stress the fact that for
extremal supergravity black holes the entropy formula is topological, that is
the entropy turns out to be a moduli independent quantity and can be written in
terms of invariants of the duality group of the supergravity theory.
