We rely on interactions between our sensory systems to help us communicate with each other and navigate our world. This multisensory integration can improve the accuracy of the sensory systems involved. However, many questions remain on how multisensory integration, specifically audiovisual integration, is mediated within the brain. In Chapter 2, we tested whether sound improves visual processing in the primary visual cortex. We found that both individual and populations of neurons encoded visual stimuli better with simultaneous auditory input. Importantly, we also found that this effect was due to sound and not mediated by sound-induced movements, an independent modulator of visual responses. These results clarify the codes underlying this tripartite interaction in this visual region. In Chapter 3, we probed the cortical circuits that support the audiovisual integration in the primary visual cortex. We found that the auditory cortex sends excitatory projections to the visual cortex, and stimulation of these fibers enhances visual response magnitude. However, suppression of this pathway failed to impair audiovisual integration in the primary visual cortex, suggesting the presence of parallel or compensatory mechanisms in this region. In Chapter 4, we explored the subcortical visual circuits that project to the inferior colliculus. We found that the superior colliculus synapses with neurons in the external shell of the inferior colliculus, and stimulation of these projections evokes activity in this auditory midbrain. However, neurons in the inferior colliculus failed to exhibit consistent responses to looming or static audiovisual stimuli, suggesting specificity in the visual tuning of these auditory neurons. Together, these results improve our knowledge of the coding and circuitry principles underlying audiovisual integration in both cortical and subcortical regions, and expand our understanding of how the brain integrates sensory information to generate our smooth perceptual experience.