  Gravitational theories with Lorentz violation must account for a number of
possible features in order to be consistent theoretically and
phenomenologically. A brief summary of these features is given here. They
include evasion of a no-go theorem, connections between spontaneous Lorentz
breaking and diffeomorphism breaking, the appearance of massless
Nambu-Goldstone modes and massive Higgs modes, and the possibility of a Higgs
mechanism in gravity.
