G. J. Whitrow (1912-2000) begins this classic exploration of the nature of time with a story about a Russian poet, visiting London before the First World War. The poet's English was not too good and when he asked a man in the street, 'Please, what is time? ' he received the response, 'But that's a philosophical question. Why ask me? '. Starting from this simple anecdote, Professor Whitrow takes us on a good-humored and wide-ranging tour of the thing that clocks keep (more or less). He discusses how our ideas of time originated; how far they are inborn in plants and animals; how time has been measured, from sundial and hourglass to the caesium clock, and whether time possesses a beginning, a direction, and an end. He coaxes the diffident layman to contemplate with pleasure the differences between cyclic, linear, biological, cosmic, and space-time, and he provides frequent diversions into fascinating topics such as the Mayan calendar, the migration of birds, the dances of bees, precognition, and the short, crowded lives of mu-mesons, particles produced by cosmic-ray showers that exist for just two millionths of a second. This reissue of the classic and authoritative What is Time? includes a new introduction by Dr. J. T. Fraser, founder of the International Society for the Study of Time, and a bibliographic essay by Dr. Fraser and Professor M. P. Soulsby of the Pennsylvania State University.