We study properties of field redefintions in QFT. First, by exploring its
effects on the S-matrix, leading to the equivalence theorem of the S-matrix.
Second, we discuss how, in Effective Field Theories, field redefinitions have
to respect the power counting. Third, we discuss applications of field redefinitions to the non-perturbative RG, making only the essential couplings scale
dependent. This becomes very important in the case of physical systems with
many couplings. We apply this to the 3 dimensional Ising model.
