Unlike auditory-only speech perception, little is known about the development of auditory-visual speech perception. Recent studies show that pre-linguistic infants perceive auditory-visual speech phonetically in the absence of any phonological experience. In addition, while an increase in visual speech influence over age is observed in English speakers, particularly between six and eight years, this is not the case in Japanese speakers. This thesis aims to investigate the factors that lead to an increase in visual speech influence in English speaking children aged between 3 and 8 years. The general hypothesis of this thesis is that age-related, language-specific factors will be related to auditory-visual speech perception. Three experiments were conducted here. Results show that in linguistically challenging periods, such as school onset and reading acquisition, there is a strong link between auditory visual and language specific speech perception, and that this link appears to help cope with new linguistic challenges. However this link does not seem to be present in adults or preschool children, for whom auditory visual speech perception is predictable from auditory speech perception ability alone. Implications of these results in relation to existing models of auditory-visual speech perception and directions for future studies are discussed. / Doctor of Philosophy (PhD)