The Laser Interferometer Space Antenna (LISA) is expected to observe and study 
the cosmic population of black hole binaries in the mass range ~10^5-10^7 solar 
masses reaching unprecedented redshifts. I will discuss the sensitivity of LISA 
to different populations of massive black holes, its ability (and limitation) 
to extract detailed information about the sources, and the implications for new 
studies in the context of astronomy, cosmology and fundamental physics.