Auroral physics is a subject that has seen considerable change and development over the past twenty years, particularly because of its importance to space research and the exploration of the near earth environment. In July 1988, a conference held at St. Johns College, Cambridge, celebrated the centenary of Sydney Chapman, the founder of the subject in its modern form, and brought together an international group of experts in the field to discuss important developments and the likely directions of future research. Auroral Physics gives a comprehensive overview of the subject, and puts forward some important new ideas.