  Whereas there exists a mathematical proof for one--site breathers stability,
and an unpublished one for two--sites breathers, the methods for determining
the stability properties of multibreathers rely in numerical computation of the
Floquet multipliers or in the weak nonlinearity approximation leading to
discrete non--linear Schr\"odinger equations. Here we present a set of
multibreather stability theorems (MST) that provides with a simple method to
determine multibreathers stability in Klein--Gordon systems. These theorems are
based in the application of degenerate perturbation theory to Aubry's band
theory. We illustrate them with several examples.
