Supermassive black holes (SMBHs) are believed to reside at
the centres of all spheroids in the local Universe; in addition, they
are linked to their host galaxies by tight correlations (as the
Mbh-sigma relation).  Surprisingly, most of these SMBHs are currently
radiatively quiescent or show very low levels of activity. The
solution to this puzzle is linked to the problem of the growth and
activity cycle of SMBHs, and the evolution of their host galaxies.
Possible explanations will be discussed, concentrating on clues derived
recently from Chandra X-ray observations.