  The most promising scenarios of baryogenesis seems to be the one through
lepton number violation. Lepton number violation through a Majarana mass of the
right-handed neutrinos can generate a lepton asymmetry of the universe when the
right-handed neutrinos decay. The left-handed neutrinos get small Majorana
masses through see-saw mechanism in these models. A triplet higgs scalar
violating lepton number explicitly through its couplings to two leptons or two
higgs doublets can also naturally give small Majorana masses to the left-handed
neutrinos and also generate a lepton asymmetry of the universe. We review both
these models of leptogenesis, where the lepton number asymmetry then gets
converted to a baryon asymmetry of the universe before the electroweak phase
transition.
