Why are most animal signals reliable ? This is the central problem for evolutionary biologists interested in signals. A number of theoretical answers have been proposed and empirical studies made, but a considerable amount of confusion still remains. The authors, one a theoretician the other a fieldworker, introduce a sense of order to this chaos. They disentangle the complex and often confusing terminology that characterises the subject, and then challenge the widely held assumption that there is only one correct explanation for signal reliability. The authors argue that the reliability of signals is maintained in several ways, relevant in different circumstances, and that biologists must learn to distinguish between them. In this book, they explain the different theories, give examples of signalling systems to which one or another theory applies, and point to the many areas where further work, both theoretical and empirical, is required.