  This is a contribution to the Proceedings of the 7th Mathematical Physics
Meeting: Summer School and Conference on Modern Mathematical Physics, held in
Belgrade 09 -- 19 September 2012.
  We give an easily accessible introduction to the ADM decomposition of the
curvature components. After that we review the basic problems associated with
attempts of constructing a viable massive gravity theory. And finally, we
present the metric formulations of ghost-free massive gravity models, and
comment on existence problem of the matrix square root.
