Lepton flavour violation in $\tau$ and $B$ meson decays

Lepton flavour violations in the charged sector can be foreseen as a consequence of the observation of neutrino oscillations, they are expected with enhanced probabilities in many extentions of the Standard Model. An overview of recent experimental searches for lepton flavour violation in $\tau$ and $B$ meson decays is presented. In particular, the results from the first search for $\tau \to \mu^+\mu^- \mu^-$ performed at an hadron collider by the LHCb experiment is described. Lepton number violations related to the possible existence of a massive Majorana neutrino are also discussed and upper limits on the branching fraction of B meson decays into hadrons and a like-sign leptons pair are presented.