  The Standard Model (SM) describes particle physics with great precision.
However, it does not account for the generation of neutrino masses, whose
nature we do not understand. Both a Dirac and a Majorana mass term could
intervene, leading to the existence of heavy partners of the SM neutrinos,
presumably more massive and nearly sterile. For suitable choices of parameters,
heavy neutrinos can also provide dark matter candidates, and generate the
observed baryon asymmetry of the universe. Heavy neutrinos can be searched for
at beam dump facilities such as the proposed SHiP experiment if their mass is
of the order of few GeV, or at high energy lepton colliders, such as the Future
$e^+e^-$ Circular Collider, FCC-ee, presently under study at CERN, for higher
masses. This contribution presents a review of the sensitivities for heavy
neutrino searches at SHiP and FCC-ee.
