The prosodic contours of mothers' speech to infants are typically exaggerated in pitch range and slower in tempo, compared to the prosody of adult-adult speech. Cross-language acoustic analyses reveal that mothers use similar pitch contours in similar communicative contexts with preverbal infants. The universality of these melodies in mothers' speech suggests that exaggerated intonation may serve important functions in the early development of communication and language. Three studies will be presented that investigate both affective and linguistic functions of intonation in speech to infants. The first study demonstrates that 4-month-old infants show greater differential emotional responsiveness to vocal expressions than to facial expressions. The second study shows that 5-month-old American infants respond with appropriate affect to positive and negative vocalizations in mothers' speech in several unfamiliar languages as well as in English. The third study investigates the role of exaggerated intonation in facilitating lexical acquisition in 12- to 18-month-old infants.