  Idealized models are used to illustrate the potential of the Laser
Interferometer Space Antenna (LISA) as a probe of the largely unknown
population of cosmologically-distant Massive Black Holes (MBHs) and as a tool
to measure their masses with unprecedented accuracy. The models suggest that
LISA will most efficiently probe a MBH population of lower mass than the one
found in bright quasars and nearby galactic nuclei. The mass spectrum of these
MBHs could constrain formation scenarios for high-redshift, low-mass galaxies.
