This review covers several topics involving renormalization group ideas. The
solution of the s-wave Kondo Hamiltonian, describing a single magnetic
impurity in a nonmagnetic metal, is explained in detail. See Sees. VII-IX.
"Block spin" methods, applied to the two dimensional Ising model, are
explained in Sec. VI. The first three sections give a relatively short review of
basic renormalization group ideas, mainly in the context of critical phenomena.
The relationship of the modern renormalization group to the older problems of
divergences in statistical mechanics and field theory and field theoretic
renormalization is discussed in Sec. IV. In Sec. V the special case of "marginal
variables" is discussed in detail, along with the relationship of the modern
renormalization group to its original formulation by Gell-Mann and Low and
others.