  We present new theoretical results on the spectrum of the quantum field
theory of the Double Sine Gordon model. This non-integrable model displays
different varieties of kink excitations and bound states thereof. Their mass
can be obtained by using a semiclassical expression of the matrix elements of
the local fields. In certain regions of the coupling-constants space the
semiclassical method provides a picture which is complementary to the one of
the Form Factor Perturbation Theory, since the two techniques give information
about the mass of different types of excitations. In other regions the two
methods are comparable, since they describe the same kind of particles.
Furthermore, the semiclassical picture is particularly suited to describe the
phenomenon of false vacuum decay, and it also accounts in a natural way the
presence of resonance states and the occurrence of a phase transition.
