Searches for Lepton Flavour Violation and Lepton Number Violation in Hadron Decays

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.