Theory of rare kaon decays

We review some recent theoretical results on rare kaon decays. Particular attention is devoted to find Standard Model tests. This is theoretically easy in $K\to \pi \nu \bar{\nu}$, while a careful study of the long distance contributions is needed for $K_{L}\to \pi ^{0}e$, $K_{L}\to \mu \bar{\mu}$ and $K\to \pi \pi \gamma$