As previously discussed, from early infancy, vocal communication most often takes the form of a give-and-take exchange. Turn-taking of this nature is very important for infants in developing the ability to imitate. Moreover, temporal patterning is neither the only nor the most salient characteristic of these interactions; in addition, caregivers prefer specific characteristics of infant vocalizations, especially those that occur in cooing vocalizations. However, what receives selective preference is not simply the infant vocalization, but actually the vocal characteristics of both partners' vocalizations. Indeed, with respect to early communication, scientists have devoted considerably more attention to adult than infant vocal quality. When an adult says “hi” and then addresses an infant by name, the “hi” is typically uttered at the highest pitch and in two syllables, with a falling intonation. Adults typically utter the infant's name with an exaggerated and rising melody. Conventionally, we know that most adults and children use this speech style when speaking to infants and, almost intuitively, we know that, given a choice, infants prefer to hear speech of this sort. In this chapter, we will address an important issue relating to this vocal interaction system. We will address the functional significance of speech style as a selective response in adults and as selectively attended to by infants.