Paul Dirac was one of the finest physicists of this century. The development of quantum mechanics began at the turn of the century, but it was Dirac who, in 1925 and 1926, brought the subject to its definitive form, creating a theory as compelling as Newton's mechanics had been.

Dirac immediately set about reconciling the quantum theory with Einstein's special theory of relativity (of 1905). The nature of the marriage between these two marvellous theories, and the fruits of that union, have been the constant preoccupation of fundamental physics from 1925 to the present day. Dirac contributed more than anyone else to this crucial enterprise, including in 1930 the prediction of the existence of antimatter.

Dirac died in 1984, and St John's College, Cambridge (Dirac's college), very generously endowed an annual lecture to be held at Cambridge University in Dirac's memory. The first two Dirac Lectures, printed here, are contrasting variations on Dirac's theme of the union of quantum theory and relativity.

Richard Feynman, in the years since the Second World War, did more than anyone else to evolve Dirac's relativistic quantum theory into a general and powerful method of making physical predictions about the interactions of particles and radiation. His work complements Dirac's in a remarkable way. His style of doing physics has been vastly influential. His lecture here, which gives some flavour of that style, expounds the physical reality underlying Dirac's prediction of antimatter.

The crowning achievement to date of the relativistic quantum theory has been the unification of electricity and magnetism on the one hand (themselves unified by Maxwell a century ago) with the weak forces of radioactive decay on the other. Steven Weinberg is one of the chief authors of this unification, in work which predicted the existence and properties of new particles (weighing as much as heavy atoms), which were subsequently triumphantly produced, precisely as predicted, at the European laboratory CERN in Geneva in 1983.