  Neutrinos are the only particles in the Standard Model of particle physics
that have only been observed with left handed chirality to date. If right
handed neutrinos exist, they would not only explain the observed neutrino
oscillations, but could also be responsible for several phenomena in cosmology,
including the baryon asymmetry of the universe, dark matter and dark radiation.
A crucial parameter in this context is their Majorana mass, which in principle
could lie anywhere between the eV scale and GUT scale. The implications for
experiments and cosmology strongly depend on the choice of the mass scale. We
review recent progress in the phenomenology of right handed neutrinos with
different masses, focusing on scenarios in which the mass is at least a keV. We
emphasise the possibility to discover heavy neutrinos that are responsible for
the baryon asymmetry of the universe via low scale leptogenesis in near future
experiments, such as LHC, BELLE II, SHiP, FCC-ee or CEPC.
