In the framework of the Standard Model (SM) of particle physics, flavour-changing-neutral-current (FCNC) decays are predicted to be rare. This makes them excellent probes of the quantum structure of the SM and sensitive to effects beyond it. First mearsurments have been accumulated at the 1st generation B-factories and Tevatron experiments in the past decade which will be improved and extended during the current run of LHCb and by the 2nd generation B-factories in the future. An introduction will be given of how rare FCNC decays can contribute to open questions of the SM and the search for physics beyond.