Daily life confronts us with information across multiple senses in parallel and we must somehow unite these sensations into a coherent interpretation of the world. MULTISENSE will explain how the human brain comes to develop this complex ability, and how certain genetically ‘pre-marked’ brains can come to experience multisensory integration in remarkable ways. Implicit associations across the senses can have a profound and sometimes surprising effect on our experiences (eg, the taste of food can be altered simply by changing its colour, its shape, or even its name). People with the neurological condition synaesthesia have cross-sensory interactions to an extreme degree (eg, they might experience tastes as colourful moving shapes in the visual field). Synaesthesia can give profound insights into normal perception because all people experience crossed sensations to an implicit degree, and often in ways that mimic synaesthetes (eg, rougher textures tend to be ‘seen’ as darker colours by synaesthetes, and are also paired to darker colours by nonsynaesthetes in sensory matching tasks). In this ambitious proposal I will examine the life-span development of multisensory integration in both synaesthetes and nonsynaesthetes, considering changes that occur throughout childhood, non-elderly adulthood, and older age. My project will unfold over 48 months across 4 inter-related Work Programmes, in which I will develop a state-of-the-art assessment tool to identify child synaesthetes, and use cognitive tests and experimental techniques to evaluate multisensory functions in synaesthetes and nonsynaesthetes across the lifespan. My questions stand at the frontier of our scientific understanding, and will develop an embryonic field of childhood synaesthesia research, and forge a wholly novel field in synaesthesia and aging. My findings will be fed back to both researchers and educational policy makers, and will frame this field for current and future academics.