Abstract In this report the status of lepton flavour conservation is reviewed. Emphasize is given on the present round of muon decay experiments as well as recent theoretical developments. The principle of lepton flavour conservation is based primarily on empirical evidence. Within the Standard Model description this observation is related to the degeneracy of the neutrino states. In most generalizations of the model the selection rule is violated at levels which may be accessible experimentally. Three major new experiments looking for forbidden muon decays are in preparation. The MEGA spectrometer, under construction at LAMPF, will push the sensitivity to the branching ratio of the decay μ → ργ to ≈ 6 × 10 −13 . At PSI the new SINDRUM II spectrometer has started to take data on coherent μ − ( A , Z ) → ρ − ( A , Z ) conversion, aiming at a sensitivity of 3 × 10 −14 . The largest improvement in sensitivity is anticipated in a renewed search for spontaneous conversion of muonium to antimuonium: μ + ρ − → μ − ρ + . The latter experiment makes use of the SINDRUM I spectrometer and should obtain a precision around 10 −3 × G F for the coupling constant.