Only rarely can the great figures in the physical sciences be persuaded to visit the antipodes. Hence it was remarkable that Professor H. Hora and Dr. J. L. Hughes were able, during a conference in Miami, to make tentative arrangements for the one surviving father of modern quantum theory to travel to Australia and New Zealand and to deliver there the outstanding lectures reproduced in this volume. Professor Dirac lectured in Christchurch, New Zealand and in Sydney, Adelaide and Canberra, Australia.