  In this talk we discuss a new approximation scheme for non-perturbative
calculations in a quantum field theory which is based on the fact that the
Schwinger equation of a quantum field model belongs to the class of singularly
perturbed equations. The self-interacting scalar field and the Gross-Neveu
model are taken as the examples and some non-perturbative solutions of an
equation for the propagator are found for these models. The application to QCD
is also discussed.
