  We approach the study of non--integrable models of two--dimensional quantum
field theory as perturbations of the integrable ones. By exploiting the
knowledge of the exact $S$-matrix and Form Factors of the integrable field
theories we obtain the first order corrections to the mass ratios, the vacuum
energy density and the $S$-matrix of the non-integrable theories. As
interesting applications of the formalism, we study the scaling region of the
Ising model in an external magnetic field at $T \sim T_c$ and the scaling
region around the minimal model $M_{2,7}$. For these models, a remarkable
agreement is observed between the theoretical predictions and the data
extracted by a numerical diagonalization of their Hamiltonian.

High Energy Physics - Theory;Condensed Matter