Theory of Kaon Physics

I review rare kaon decays. I introduce the flavor problem and p ossible solutions. Very rare kaon decays like $K \to \pi \nu \bar{\nu}$ are very important to this purpose but also $K \to \pi l^+ l^-$ . A new interesting channel is $K \to \pi \pi ee$. Chiral dynamics is important to disentangle short distanc e effects. We discuss also the decays $K^0 \to \pi^+ \pi^-$ , which have received recently some attention due to the measurement by LHCB.therefore we will study also $K_L \to \pi^0 e^+ e^- , K^+ \to \pi^+ \pi^0 \gamma , K^+ \to \pi^+ \pi^0 e^+ e^-$ and related channels