  In this paper we discuss massive gravity in Minkowski space via gravitational
Higgs mechanism, which provides a non-perturbative definition thereof. Using
this non-perturbative definition, we address the issue of unitarity by studying
the full nonlinear Hamiltonian for the relevant metric degrees of freedom.
While perturbatively unitarity is not evident, we argue that no negative norm
state is present in the full nonlinear theory.
