The non-zero neutrino masses and angles, observed in neutrino oscillation experiments, are currently the only signal of physics beyond the standard model, that has been confirmed in controlled laboratory experiments. The origin of neutrino masses, however, is so far not understood. If neutrinos are Majorana particles, signals at colliders could appear, if the scale of lepton number violation is not to far from the electro-weak scale. In this talk, I will briefly review some recent work on displaced vertex signals, before discussing lepton number violation in beyond-minimal seesaw models of neutrino mass.