Abstract:Two very different problems that can be studied by renormalization group methods are discussed with the aim of showing the conceptual unity that renormalization group has introduced in some areas of theoretical Physics. The two problems are: the ground state theory of a one dimensional quantum Fermi liquid and the existence of quasi periodic motions in classical mechanical systems close to integrable ones. I summarize here the main ideas and show that the two treatments, although completely independent of each other, are strikingly similar. §1. Introduction. . In all such examples there is a basic difficulty to overcome: namely the samples of the fields can be unboundedly large: this does not destroy the method because such large values have extremely small probability, [Ga85]. The necessity of a different treatment of the large and the small field values hides, to some extent, the intrinsic simplicity and elegance of the approach: unnecessarily so as the end result is that one can essentially ignore (to the extent that it is not even mentioned in most application oriented discussions) the large field values and treat the renormalization problem perturbatively, as if the large fields were not possible.Here I shall discuss two (non trivial) problems in which the large field difficulties are not at all present, and the theory leads to a convergent perturbative solution of the problem (unlike the the above mentioned classical cases, in which the perturbation expansion cannot be analytic in the perturbation parameter). The problems are:(1) the theory of the ground state of a system of (spinless, for simplicity) fermions inThe two problems will be treated independently, for completeness, although it will appear that they are closely related. Since the discussion of problem (1) is quite technical we summarize it at the end (in §3) in a form that shows the generality of the method that will then be applied to the problem (2) in §4. The analysis of the above examples suggests methods to study and solve several problems in the theory of rapidly perturbed quasi periodic unstable motion, [Ga95], [GGM99]: but for brevity we shall only refer to the literature for such applications.