  In the Standard Model of particle physics, lepton flavour and lepton number
are conserved quantities although no fundamental symmetry demands their
conservation. I present recent results of searches for lepton flavour and
lepton number violating hadron decays measured at the B factories and LHCb.
  In addition, the LHCb collaboration has recently performed a search for the
lepton flavour violating decay \tau^- \to \mu^-\mu^-\mu^+. The obtained upper
exclusion limit, that has been presented in this talk for the first time, is of
the same order of magnitude as those observed at the B factories. This is the
first search for a lepton flavour violating \tau decay at a hadron collider.
