  We give an introduction to massive spin-2 theories and the problem of their
non-linear completion. We review the Boulware-Deser ghost problem and two ways
to circumvent classic no-go theorems. In turn, massive spin-2 theories are not
uniquely defined. In the case of truncated theories, we show that the
Boulware-Deser ghost may only be avoided if the derivative structure of the
theory is not tuned to be Einsteinian.
