This dissertation addressed important questions regarding audiovisual (AV) perception. Study 1 revealed that AV speech perception modulated auditory processes, whereas AV non-speech perception affected visual processes. Interestingly, stimulus identification improved, yet fewer neural resources, as reflected in smaller event-related potentials, were recruited, indicating that AV perception led to multisensory efficiency. Also, AV interaction effects were observed of early and late stages, demonstrating that multisensory integration involved a neural network. Study 1 showed that multisensory efficiency is a common principle in AV speech and non-speech stimulus recognition, yet it is reflected in different modalities, possibly due to sensory dominance of a given task. Study 2 extended our understanding of multisensory interaction by investigating electrophysiological processes of AV speech perception in noise and whether those differ between younger and older adults. Both groups revealed multisensory efficiency. Behavioural performance improved while the auditory N1 amplitude was reduced during AV relative to unisensory speech perception. This amplitude reduction could be due to visual speech cues providing complementary information, therefore reducing processing demands for the auditory system. AV speech stimuli also led to an N1 latency shift, suggesting that auditory processing was faster during AV than during unisensory trials. This shift was more pronounced in older than in younger adults, indicating that older adults made more effective use of visual speech. Finally, auditory functioning predicted the degree of the N1 latency shift, which is consistent with the inverse effectiveness hypothesis which argues that the less effective the unisensory perception was, the larger was the benefit derived from AV speech cues. These results suggest that older adults were better "lip/speech" integrators than younger adults, possibly to compensate for age-related sensory deficiencies. Multisensory efficiency was evident in younger and older adults but it might be particularly relevant for older adults. If visual speech cues could alleviate sensory perceptual loads, the remaining neural resources could be allocated to higher level cognitive functions. This dissertation adds further support to the notion of multisensory interaction modulating sensory-specific processes and it introduces the concept of multisensory efficiency as potential principle underlying AV speech and non-speech perception
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