In recent years there has been a rapid increase in the understanding of communication between animals and this is perhaps especially true of bio-acoustics. In the last 35 years a completely new branch of bio acoustics, involving ultrasounds, has been made possible by technical developments that now allow these inaudible sounds to be detected and studied. This subject has a personal fascination for the authors, perhaps because of the novelty of 'listening in' to these previously unknown sig nals, perhaps because of the wide variety of ways in which different animals use them. Many studies of different aspects of animal ultrasound have now been published and a review of them all seems to be timely. Ultrasound is is biologically arbitrary; other animals defined in human terms and may produce similar signals at lower frequencies for similar purposes. This book attempts to be comprehensive but the limits of the subject are rather difficult to define. It should be read in conjunction with other books on audible bio-acoustics. Each chapter has been written and may be read as a separate entity, although there is considerable cross-referencing. Chapters 1 and 2 form a common introduction and may help in understanding the later sections. The Appendix is not essential but is included for those who may be interested in the quanti tative aspects of the echo-location phenomena described in Chapters 3 and 8.