In the presence of magnetic impurities, the resistance of a metal shows an unexpected low temperature minimum: this is the so called Kondo effect. Since its first experimental observation, in 1934, the Kondo effect has been extensively studied trough the years. In this seminar I will briefly review the milestones of the theoretical understanding of Kondo effect. I will start from the early approaches of Anderson and then discuss the problem of low temperature logarithmic divergences emerging in the perturbative analysis of Kondo Hamiltonian. Then I will discuss perturbative and nonperturbative renormalization schemes developed in order to regularize such divergences: in particular, I will focus on the simple Anderson's ``Poor Man scaling'' approach and then only schetch the Numerical Renormalization Group solution proposed by Wilson, whose detailed discussion will be given in the technical lecture. Recently, a renewed interest in Kondo physics is arriving from nanophysics. In fact, single channel, multi channel and orbital Kondo effect have been observed in many kinds of nanostructures.