  We study the perspective to observe lepton number violating signatures from
heavy Majorana neutrino decays at colliders in view of the requirement to
explain the light neutrino masses via the seesaw mechanism. In the minimal
model with only two heavy neutrinos and in the $\nu$MSM one can identify three
distinct regions in the mass-mixing plane. For Majorana masses above the
electroweak scale the branching ratio for lepton number violating processes at
the LHC is generically suppressed. For masses well below the electroweak scale
that are probed in displaced vertex searches or at fixed target experiments
lepton number violation is the rule and can only be avoided at the cost of fine
tuning. In between there is a mass regime where both possibilities coexist. In
models with more than two heavy neutrinos the larger parameter space allows for
more freedom, but our results remain qualitatively correct unless there is a
mass degeneracy amongst more than two of the heavy neutrinos.
