Paul Adrian Maurice Dirac, one of the greatest physicists of the twentieth century, died in 1984. Dirac's college, St John's of Cambridge, generously endowed annual lectures to be held at Cambridge University in his memory. This 1990 volume includes an expanded version of the third Dirac Memorial Lecture presented by Abdus Salam, in addition to two previously unpublished lectures by Paul Dirac and by Werner Heisenberg.