In recent years there has been an enormous progress in low-dimensional quantum field theory. The most important results concern the conformal properties of the critical points of the Renormalization Group and the scaling region nearby. In this respect a crucial role is played by integrable deformations of Conformal Field Theories, which can be solved using bootstrap methods coming from S-matrix theory. In these lectures I present the Form-Factor Approach to the computation of correlation functions. Non-perturbative methods of both Conformal and Integrable Field Theories find remarkable applications in low-dimensional quantum systems.