  LISA is a planned space-based gravitational-wave (GW) detector that would be
sensitive to waves from low-frequency sources, in the band of roughly (0.03 -
0.1) mHz < f < 0.1 Hz. This is expected to be an extremely rich chunk of the GW
spectrum -- observing these waves will provide a unique view of dynamical
processes in astrophysics. Here we give a quick survey of some key LISA sources
and what GWs can uniquely teach us about these sources. Particularly noteworthy
science which is highlighted here is the potential for LISA to track the
moderate to high redshift evolution of black hole masses and spins through the
measurement of GWs generated from massive black hole binaries (which in turn
form by the merger of galaxies and protogalaxies). Measurement of these binary
black hole waves has the potential to determine the masses and spins of the
constituent black holes with percent-level accuracy or better, providing a
unique high-precision probe of an aspect of early structure growth. This
article is based on the "Astrophysics and Relativity using LISA" talk given by
the author at the Seventh Edoardo Amaldi Conference on Gravitational Waves; it
is largely an updating of the author's writeup of a talk given at the Sixth
International LISA Symposium.
