Rare decays are quite sensitive1 to physics beyond the Standard Model (SM), due to the effects of virtual, new, heavy particles (i.e., a fourth generation, SUSY, charged Higgs, E6 exotic fermions,…). These new contributions can either enhance or suppress (sometimes quite substantially) the rates for rare processes compared to the corresponding SM predictions. Current experiments are already closing in on the SM rates for some processes and are starting to be able to probe the possible contributions from new physics. With the advent of K and B factories, as well as with detector improvements, future experiments expect to actually observe the predicted SM rate for several decays and may be able to rule out some extended models. In this case, rare processes may hold a rare advantage over collider physics, as they will be able to explore new higher energy regimes in the near future.