  There is a strong experimental program searching for massive sterile
neutrinos. Here we focus on the heavy regime above the GeV scale, where their
existence can be probed in either high-energy colliders or in high-precision
facilities. We first review the current experimental status at colliders,
showing that the LHC already improves LEP results for mixings to electrons and
muons, while mixings to taus remain challenging at a hadron collider. We also
discuss the importance of exploring both lepton number violating and conserving
signals, as well as different flavor channels. Finally, we present the latest
global analysis of electroweak precision and flavor observables, showing that
the intensity frontier provides the strongest constraints for heavy neutrino
masses above the electroweak scale.
