  We briefly review and compare three methods (one perturbative, one based on
Ward Identities and one non-perturbative) for the calculation of the
renormalization constants of lattice operators. The following results are
presented: (a) non perturbative renormalization of the operators with light
quarks; (b) the renormalization constants with a heavy (charm) quark mass and
its KLM improvement; (c) the non perturbative determination of the mixing of
the $\Delta S = 2$ operator.
