Research has shown that both adults and children will imitate acoustic properties of the speech around them. In fact, studies on adults have shown that this convergence occurs even when the subject simply sees, but does not hear, the interlocutor. Not only does visual speech elicit imitation on its own, but also imitation is greater for audiovisual speech than for auditory-only speech. However, these studies on audiovisual imitation have not looked at which properties of the speech are better imitated with the addition of the visual cues. In this dissertation, I compare imitation in the auditory and audiovisual modalities to determine if audiovisual presentation (a) enhances the uptake of specific acoustic-phonetic cues, such as vowel formants, or (b) non-criterial information (f0 and duration). For this I examine how closely children and adults imitate the productions of English-like and foreign vowels presented auditorily vs. audiovisually. Additionally, I attempt to determine how different speaking registers (adult-directed speech and child-directed speech) can aid in imitation. Adult participants in this study showed greater imitation in the audio-visual modality, as has been shown previously. The increase in imitation was shown globally, across all different types of measures (acoustic-phonetic cues and non-criterial measures such as f0 and duration). The child-directed register as well facilitated adult imitation in a global manner. In contrast, child participants showed equivocal findings of increased imitation in the audiovisual modality and child-directed register on non-criterial measures such as f0 and duration. However, for the acoustic-phonetic cues, they showed more uniform increases of imitation in the audiovisual modality and child-directed register. The results of these experiments add to our understanding of how the visual modality is relevant in imitation, language development, and second-language learning. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]