Right-handed neutrinos are among the best-motivated extensions beyond the Standard Model.
When they are subject to a protective symmetry they can have large mixings with the active neutrinos and masses at the electroweak scale. The effects of these particles can be studied in many different ways at the Future electron-positron Collider. In particular, the modifications of the weak currents due to neutrino mixing can be studied via precision measurements of electroweak observables. Searches for the spectacular signature of long-lived heavy neutrinos that decay with macroscopic displacement allow to almost test the type-I seesaw prediction, where the neutrino mixing is fixed by the ratio of the light and the heavy neutrino mass scale. Moreover, the new Yukawa interaction allows for modified Higgs boson production and decay modes. The FCC-ee has great discovery prospects and it is one of the most promising colliders to test this class of models.