Rare K and B decays in the standard model and supersymmetry

We discuss rare decays as probes of physics beyond the Standard Model. Afte r quickly reviewing the SM predictions fur rare K and B decays, we discuss SUSY contributions to these processes in two extreme scenarios: the MSSM wi th minimal flavour and CP violation, and the most general minimal SUSY exte nsion of the SM. We stress how rare decays are in principle able to discrim inate between various scenarios of new physics.