  Owing to the strong suppression of flavour-changing neutral-current
transitions in the Standard Model, rare kaon decays constitute a superb tool to
constrain hypothetical new-physics interactions. At the same time, they provide
many interesting tests of the Standard Model itself, being sensitive both to
short-distance electroweak scales and to the long-distance QCD dynamics. A
brief overview of the current status is presented. The Standard Model
prediction for the direct CP-violating ratio $\varepsilon'/\varepsilon$ is also
discussed.
