  We review the present electroweak precision data constraints on the mediators
of the three types of see-saw mechanisms. Except in the see-saw mechanism of
type I, with the heavy neutrino singlets being mainly produced through their
mixing with the Standard Model leptons, LHC will be able to discover or put
limits on new scalar (see-saw of type II) and lepton (see-saw of type III)
triplets near the TeV. If discovered, it may be possible in the simplest models
to measure the light neutrino mass and mixing properties that neutrino
oscillation experiments are insensitive to.
