Chapter 15 introduces the notion of the scaling region near the critical points, identified by the deformations of the critical action by means of the relevant operators. The renormalization group flows that originate from these deformations are subjected to important constraints, which can be expressed in terms of sum-rules. This chapter also discusses the nature of the perturbative series based on the conformal theories. Further, it describes how the analysis of the off-critical theories poses a series of interesting questions, and also covers ultraviolet divergences, structure constants, the two-point function of the Yang–Lee model, the RG and β-functions and the c-theorem.