Rare Decays

This review presents recent results on key measurements of rare $B$ and $\tau$ decays performed at the LHC and the TeVatron. Its main focus lies on recent measurements of the decay $B^0_{(S)} \to \mu^+ \mu^-$ at the two colliders and of angular observables for the decay $B^0 \to K^{*0} \mu^+ mu^-$ at the LHC experiment ATLAS, CMS and LHCb. Furthermore, the branching fraction measurement at low dilepton masses in the channel $B^0 \to K^{*0} e^+ e^-$ from LHCb and results on the decay $B^\pm \to \pi^+ \pi^-$ as well as on lepton-flavour- and baryon-number-violating rare decays of $\tau$ leptons are discussed.