Synaestheia is an inherited neurological condition that gives rise to a kind of ‘merging of the senses’. For example, synaesthetes might ‘see’ colours when they hear music, or experience tastes in the mouth when they read words. One particularly common variant is experiencing colours when reading letters or numbers, and this variant of the condition – known as grapheme-colour synaesthesia – is found in around 1 in 100 people. What are the experiences of synaesthetes, and how do these unusual experiences develop during childhood? How do they impact on schooling and early life development and how do adult synaesthetes navigate their multisensory worlds? I will explore the nature of these cross-sensory experiences and ask what they might also tell us about sensory processing in the population at large. I’ll describe what I have learned from the scientific research carried out at my Synaesthesia and Sensory Integration lab over the last decade, and how synaesthesia might open novel ways of understanding creativity, perception and the very nature of reality.