  We compute non-perturbatively the renormalization constants of quark
bilinears on the lattice in the quenched approximation at three values of the
coupling beta=6/g_0^2=6.0,6.2,6.4 using both the Wilson and the tree-level
improved SW-Clover fermion action. We perform a Renormalization Group analysis
at the next-to-next-to-leading order and compute Renormalization Group
invariant values for the constants. The results are applied to obtain a fully
non-perturbative estimate of the vector and pseudoscalar decay constants.
