  We review some of the recent results which can be useful for better
understanding of the problem of stability of vacuum and in general classical
solutions in higher derivative quantum gravity. The fourth derivative terms in
the purely gravitational vacuum sector are requested by renormalizability
already in both semiclassical and complete quantum gravity theories. However,
because of these terms the spectrum of the theory has unphysical ghost states
which jeopardize the stability of classical solutions. At the quantum level
ghosts violate unitarity, and and thus ghosts look incompatible with the
consistency of the theory. The `dominating' or `standard' approach is to treat
higher derivative terms as small perturbations at low energies. Such an
effective theory is supposed to glue with an unknown fundamental theory in the
high energy limit. We argue that the perspectives for such a scenario are not
clear, to say the least. On the other hand, recently there was certain progress
in understanding physical conditions which can make ghosts not offensive. We
survey these results and discuss the properties of the unknown fundamental
theory which can provide these conditions satisfied.
