Purpose: The function of child-directed speech has been debated for decades. This study examined the perceptual and acoustic characteristics of child- and adult-directed Cantonese tones to test the hyperarticulation and prosodic hypotheses that have been proposed to account for the acoustic modifications in child-directed speech. Method: Sixty-two mother-child dyads participated in the study. The mothers verbally labeled 30 pictures in monosyllabic isolated words and in the final position of a carrier sentence to the experimenter and their 1- to 5-year-old children. The 8,634 adult- and child-directed productions were low-pass filtered to eliminate lexical information and presented to 5 judges for tone identification. Acoustic analysis was performed on the productions. Results: Acoustically, child-directed tones were produced with an elevated pitch, and the pitch level decreased as the child's age increased. Acoustic contrasts between phonetically similar and more confusing tones were not enhanced in child-directed speech, and unexpectedly, child-directed tones were identified with a lower accuracy than adult-directed tones. The perceptual errors of child-directed tones mirrored the errors found in identifying tones excised from sentence-final position, which had a pitch-lowering effect on the tones. The lower perceptual accuracy, the lack of enhanced acoustic contrasts in confusing tone pairs, and the similarities in the error patterns in identifying tones in child-directed speech and tones in utterance-final position suggest that the acoustic modifications in child-directed tones are prosodic effects serving pragmatic purposes. Conclusion: The findings reject the hyperarticulation hypothesis and support the prosodic hypothesis of child-directed speech.