  The systematic approach for the off-perturbative calculations in disordered
systems is developed. The proposed scheme is applied for the random temperature
and the random field ferromagnetic Ising models. It is shown that away from the
critical point, in the paramagnetic phase of the random temperature model, and
in the ferromagnetic phase of the random field one, the free energy contains
non-analytic contributions which have the form of essential singularities. It
is demonstrated that these contributions appear due to localized in space
instanton-like excitations.
