  We extend form-factor perturbation theory to non--integrable deformations of
massless integrable models, in order to address the problem of mass generation
in such systems. With respect to the standard renormalisation group analysis
this approach is more suitable for studying the particle content of the
perturbed theory. Analogously to the massive case, interesting information can
be obtained already at first order, such as the identification of the operators
which create a mass gap and those which induce the confinement of the massless
particles in the perturbed theory.
