Sexual selection by female choice has been defined operationally as a change in male fitness due to variance in the number of mates, and studies of population biology have demonstrated this effect clearly. We argue that sexual selection is a richer phenomenon than this narrow definition implies, involving the interaction of sensory and perceptual mechanisms with signals, all of which can be influenced by an organism's hormonal and experiential milieu. The interaction between signals and receivers is further modulated by physiological processes, social context, and the physical environment. Thus, an approach to sexual selection that integrates mechanistic factors is essential for comprehensive understanding. Merging population biology with mechanistic studies, however, might not be sufficient. This is because the precise forms of the receivers and signals are the products not only of selection on current variability, but also of their evolutionary histories. We cannot imagine how researchers can hope to understand not only the "message" conveyed by a signal but the particular phenotypes that convey and receive this message without reference to history. We urge that our field advance beyond operational definitions and toward an organismal and historical understanding of the processes and mechanisms that underlie sexual selection.