  I review the origin of the lepton asymmetry which is converted to a baryon
excess at the electroweak scale. This scenario becomes more attractive if we
can relate it to other physical phenomena. For this reason I elaborate on the
conditions of the early universe which lead to a sizable lepton asymmetry. Then
I describe methods and models which relate the low energy parameters of
neutrinos to the high energy (cosmological) CP-violation and to neutrinoless
double beta-decay.
