The series of Oskar Klein Memorial Lectures is a must-read for those keenly involved or simply interested in exploring the many fascinating aspects of Physics. This volume presents two landmark lectures given by Hans Bethe in October 1990 and Alan H. Guth in June 1991 under the series of Oskar Klein Memorial Lectures. Hans Bethe's lectures dealt with two themes: the astrophysical importance of neutrinos in supernova outbursts and a theoretical account of neutrinos through observations of the neutrino flux from the centre of the sun. Anyone interested in understanding the processes involved in the collapse and explosion of a large star would certainly find this book enlightening. Alan H. Guth's lecture dealt with the various aspects of the origin of the universe ? a topic which never fails to intrigue. The originator of the inflation scenario for the Big Bang theory, Guth has included his latest observations on the COBE satellite and their theoretical interpretation in this lecture. Anyone wishing to grasp the essentials of these ideas, will find in Guth's lecture a wealth of knowledge. This volume also presents for the first time in English the original derivation of the Klein-Nishima formula for Compton scattering and an account of the ?Klein Paradox?. A special study reveals interesting facts on the callaboration between Oskar Klein and Yoshio Nishima in 1928 and further, surprising facts on the treatment by the Nobel Committee for Physics of the prize to A H Compton in 1927. Some translated autobiographic texts have also been included to acquaint the reader with Klein's interest in cosmology and his attempts to find the driving force behind the expanding system of galaxies, what Klein termed the Meta-galaxy.