Introduction: Perspectives on Communication (D.H. Owings et al.). The Behavior of Communicating, after Twenty Years (W.J. Smith). Conspicuousness and Diversity in Animal Signals (M.S. Dawkins, T. Guilford). What Is the Function of Song Learning in Songbirds? (M.D. Beecher et al.). Referential Signals (C.S. Evans). Sexual Selection and the Evolution of Advertisement Signals (M.D. Greenfield). Determinants of Conflict Outcomes (W.J. Hamilton III, J.W. McNutt). Incidental Emissions, Fortuitous Effects, and the Origins of Communication (M.S. Blumberg, J.R. Alberts). Studying How Cetaceans Use Sound to Explore Their Environment (P.L. Tyack). An AffectConditioning Model of Nonhuman Primate Vocal Signaling (M.J. Owren, D. Rendall). Speech Acts and Animal Signals (A.G. Horn). The Role of Information in Communication: An Assessment/Management Approach (D.H. Owings, E.S. Morton). Communication and Natural Design (N.S. Thompson).