Paul Adrien Maurice Dirac was one of the founders of quantum theory. He is numbered alongside Newton, Maxwell and Einstein as one of the greatest physicists of all time. Together the lectures in this volume, originally presented on the occasion of the dedication ceremony for a plaque commemorating Dirac in Westminster Abbey, give a unique insight into the relationship between Dirac's character and his scientific achievements. The text begins with the dedication address given by Stephen Hawking at the ceremony. Then Abraham Pais describes Dirac as a person and his approach to his work. Maurice Jacob explains how Dirac was led to introduce the concept of antimatter, and its central role in modern particle physics and cosmology, followed by an account by David Olive of the origin and enduring influence of Dirac's work on magnetic monopoles. Finally, Sir Michael Atiyah explains the deep and widespread significance of the Dirac equation in mathematics.