Our lives are guided chiefly by sound and sight. We react to and learn from what we hear and what we see. Although we also obtain vital information from what we taste and what we smell, our reliance on these chemical senses is generally regarded as being of lesser importance. If we consider the entire biotic world, however, we realize that this hierarchy of information-gathering senses is far from universal. From microbes to mammals, all organisms detect selected chemical cues in their environment, and all respond to a wide variety of molecular messages behaviorally or developmentally. The interplay of chemical cues and the chemical receptors that detect and transduce these cues, initiating adoptive responses to them, results in a vastly complex communicative system that is fundamental to the fabric of life. It is clear that organisms depend on chemical signals not only to lure mates, associate with symbionts, deter enemies, and fend off pathogens, but also for a myriad of other functions. Nevertheless, it is extremely difficult to establish the roles that specific secondary (and occasionally primary) metabolites play in the world of chemical communication, particularly because deciphering chemical language requires expertise in a wide range of chemical and biological disciplines. This symposium will provide accounts of some of the most recent research into how and with what results microbes, plants, invertebrates, and vertebrates speak to one another chemically.