Why are animal signals reliable? This is the central problem for evolutionary biologists interested in signals. Of course, not all signals are reliable; but most are, otherwise receivers of signals would ignore them. A number of theoretical answers have been proposed and empirical studies made, but there still remains a considerable amount of confusion. The authors, one a theoretician the other a fieldworker, introduce a sense of order to this chaos. A significant cause of confusion has been the tendency for different researchers to use either the same term with different meanings, or different terms with the same meaning. The authors attempt to clarify these differences. A second cause of confusion has arisen because many biologists continue to assume 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.