  I overview the current understanding of the evolution of massive black hole
(MBH) binaries in the center of the host stellar system. One of the main
questions is whether the stellar dynamical effect can make the MBH binary hard
enough that they can merge through gravitational wave radiation. So far,
theories and numerical simulations suggested otherwize, since the hardening
timescale becomes very long once "loss cone" is depleted. I'll present the
result of recent simulations on this hardening timescale, and discuss its
implication on the formation history of massive black holes.
