Non-perturbative renormalization in kaon decays
  We discuss the application of the MPSTV non-perturbative method \cite{NPM} to the operators relevant to kaon decays. This enables us to reappraise the long-standing question of the $\Delta I=1/2$ rule, which involves power-divergent subtractions that cannot be evaluated in perturbation theory. We also study the mixing with dimension-six operators and discuss its implications to the chiral behaviour of the $B_K$ parameter. 
A. Donini, V. Gim\'enez, G. Martinelli, G.C. Rossi, M. Talevi, M.Testa   and A. Vladikas