Numerous studies have investigated selective forces that appear to influence the evolution of acoustic communication systems. I review a number of constraints on evolution in these systems. A species' history is of undeniable importance in the analysis of any trait. However, studies have given little or no attention to phylogenetic patterns of acoustic signals. This precludes the identification of phylogenetic constraints, and should be viewed as a serious constraint on our ability to understand how communication systems evolve. Morphological constraints influence the energetic efficiency of acoustic communication. To maximize transmission distance of signals used in long-range communication, the animal's morphology favors signals with high frequencies. Thus morphology acts in opposition to properties of the environment, which favor low frequencies for use in long-range communication. Sensory receptors also play an important role in the evolution of acoustic signals. There is significant variation in the frequency range to which an inner-ear organ of the frog is sensitive. Different lineages of frogs are characterized by ear organs with different ranges of sensitivity. This variation should influence the frequency range over which calls evolved and, as a consequence, might have influenced the rate at which different lineages of anurans speciate.