[en] Weak decays are significantly altered by effects of strong interaction. In the K system these corrections give rise to ΔI = 1/2 rule, and have measurable effects on rare weak process like K→πν bar ν, K→πe⁺e^- and var-epsilon '/var-epsilon. The situation in the K system is briefly reviewed. Strong interaction effects are also important in D and B system. A possible mechanism for D⁺-D⁰ life-time difference is reviewed and then rare B processes are dealt with extensively. Electromagnetic penguin diagrams are enhanced by QCD corrections, giving rise to process like B→K^*γ and B→Ke⁺e^-. Process mediated by gluon penguins are also discussed. 24 refs., 8 figs