  This paper reviews some aspects of the physics of neutrinos, in particular
neutrino masses and the issue of Dirac versus Majorana neutrinos. The see-saw
mechanism is described and it is argued that the Majorana nature of neutrinos
can be tested by measuring the invisible decays of the Higgs particle, as its
decay into neutrinos is determined by their Yukawa couplings, i.e. the Dirac
masses, rather than the physical Majorana masses. The measurement would allow
us to probe the scale M of the large Majorana masses for right-handed singlet
neutrinos. The optimal machine for performing such a measurement would be a
future electron-positron collider.
