I present the theoretical basis for Leptogenesis and its implications for the structure of the universe. It is suggested that density fluctuations grow during the transition period and remnants of this effect should be sought in the universe. The relation between theories with Majorana neutrinos and low energy phenomena, including oscillations, advanced considerably during the past two years with a consistent picture developed in several models.

9 pages, 3 figures.To appear in the proceedings of The IXth International Symposium on Particles, Strings and Cosmology at the Tata Institute of Fundamental Research, Mumbai (Bombay), India, during 3-8 January 2003