Leptogenesis: Lepton Number Violation in Cosmology and Particle Physics

Lepton number nonconservation which may have occurred in the early universe is a key to understand how the baryon asymmetric universe is realized in the present epoch. We shall review the fundamental theory towards this goal. The grave problem of gravitino overproduction, a problem common both to leptogenesis and GUT baryogenesis once thermal abundance of agent particles is assumed, is stressed, and two possible ways out of this difficulty are mentioned. How lepton number violation along with CP violation might be detected in terrestrial laboratories is also briefly mentioned.