GeoffGoldspink and I edited a book, Mechanics and Energetics of Animal Locomotion, which was published by Chapman and Hall in 1977. It dealt at an advanced level with all aspects of animal locomotion, emphasizing particularly the topics of then current research. Since then there have been more elementary books on the subject, including my own Locomotion of Animals (Blackie 1982), and specialized books on such topics as swimming and flight, but (despite very substantial progress in research) there has been no advanced book covering the whole range of animal locomotion. It seemed to me and to Professor Gilles (editor ofthis series) that a new book was needed. Plainly, a book of this length cannot contain everything that is known about animal locomotion. We have not attempted to make it encyclopedic, but have tried to show where the study of animal locomotion stands now, and where it is going. Older books remain useful as sources of long-established information.