Massive Black Holes Binaries (MBHBs) are becoming an hot topic
both in theoretical and in observational astrophysics.
Robust hierarchical models for structure formation predict
they form in large number during cosmic history,
and a cosmological population of MBHBs may leave a
number of unambiguous observational signatures.
MBHBs strongly interact with their stellar environment.
I will present an hybrid model for the description of
this interaction and I will show how the presence
of a MBHB in a galactic bulge should leave a peculiar signature
on the stellar population.
MBHBs are natural gravitational waves (GWs) sources. Implementing a
specific model for MBHB formation and evolution, I will show
how the planned Laser Interferometer Space Antenna (LISA),
scheduled for launch in 2013, will be able to detect these objects
and constrain their formation and evolution.
Detections of GWs from MBHBs will be an even more powerful tool in
improving our knowledge of structure formation and evolution if
accompanied by the identification of an electromagnetic counterpart  (EMC).
I'll focus on the challenge of finding counterparts of GW events
overviewing the reliability of different EMC candidates.