Finite Volume Spectrum of Sine-Gordon Model and its Restrictions

In this thesis, we review recent progresses on Nonlinear Integral Equation approach to finite size effects in two dimensional integrable quantum field theories, with emphasis to Sine-Gordon/Massive Thirring model and restrictions to minimal models perturbed by ${\mathrm{\Phi }}_{1,3}$. Exact calculations of the dependence of energy levels on the size are presented for vacuum and many excited states.