  Massive black hole binary systems are among the most interesting sources for
the Laser Interferometer Space Antenna (LISA); gravitational radiation emitted
during the last year of in-spiral could be detectable with a very large
signal-to-noise ratio for sources at cosmological distance. Here we discuss the
impact of LISA for astronomy and cosmology; we review our present understanding
of the relevant issues, and highlight open problems that deserve further
investigations.
