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.