Primate vocalizations reveal considerable variation not only between but also within species. For instance, acoustic variation has been shown to occur with the context in which the signal is given, or the emotional state of the signaller. One particularly important question is whether vocalizations reveal honest information about sender characteristics such as size, reproductive state or fighting ability. When signaler and receiver have different interests, there will always be an incentive for signalers to exaggerate or conceal their true state. Reliable signals are maintained whenever there is some type of cost or constraint on signalers that makes cheating a less optimal strategy than revealing the truth. The costs that constrain signaler behavior take a variety of forms and may, for example, be physiological, ecological, or social. We use state-of-the art acoustic analyses and a variety of multivariate statistical techniques to analyze the information content of primate vocalizations. Our aim is to explore the mechanisms underlying sound production. For instance, we examine how voice characteristics are linked to fighting ability, body size, and hormonal state. Such analyses allow us to formulate hypotheses about the costs associated with signal production, the function of such signals, and the constraints operating on the evolution of primate vocalizations.