In 1963 a group of physicists, mathematicians and philosophers of science assembled in Cornell to discuss the arrow of time. One of them was Richard Feynman, who drew attention to his comments in the published discussions by insisting that they not be attributed to him. (They appeared as the remarks of "Mr. X".) Twenty eight years later Feynman was gone, but the mysteries of time asymmetry in physics remained as deep as ever. At the end of September, 1991, forty five physicists and mathematicians assembled in Mazagon, Spain, for a second meeting on the arrow of time. This book is the proceedings of that meeting.