QCD and Weak Interactions of Light Quarks

This review contains an overview of strong interaction effects in weak decays starting with a historical introduction. It contains a short overview of semileptonic decays and their relevance for measuring CKM matrix elements. The main part is devoted to the theoretical calculation on nonleptonic matrix elements relevant for $K^0$-$\overline{K^0}$ mixing and $K\to \pi \pi$ decays. It concludes with a short summary of rare kaon decays.