  During a large period of time, the anomalous baryon number violating
interactions are in equilibrium, when the $(B+L)$ asymmetry is washed out. If
there is any lepton number violation during this period, that will also erase
the $(B-L)$ asymmetry. As a result, survival of the baryon asymmetry of the
universe pose strong constraints on lepton number violating interactions. We
review here the constraints on the left-handed Majorana neutrino masses arising
from this survival requirement of the baryon asymmetry of the universe. We then
briefly review models of leptogenesis, where lepton number violation is used to
generate a baryon asymmetry of the universe and hence the constraints on the
Majorana neutrino mass is relaxed.
