  In this thesis, we describe some recent results obtained in the analysis of
two-dimensional quantum field theories by means of semiclassical techniques.
These achievements represent a natural development of the non-perturbative
studies performed in the past years for conformally invariant and integrable
theories, which have led to analytical predictions for several measurable
quantities in the universality classes of statistical systems. Here we propose
a semiclassical method to control analytically the spectrum and the finite-size
effects in both integrable and non-integrable theories. The techniques used are
appropriate generalizations of the ones introduced in seminal works during the
Seventies by Dashen, Hasslacher and Neveu and by Goldstone and Jackiw. Their
approaches, which do not require integrability and therefore can be applied to
a large class of systems, are best suited to deal with those quantum field
theories characterized by a non-linear interaction potential with different
degenerate minima. In fact, these systems display kink excitations which
generally have a large mass in the small coupling regime. Under these
circumstances, although the results obtained are based on a small coupling
assumption, they are nevertheless non-perturbative, since the kink backgrounds
around which the semiclassical expansion is performed are non-perturbative too.
