Synaesthesia is, in the words of the cognitive neuroscientist Cytowic, a strange sensory blending. Synaesthetes report seeing colours when hearing sounds or proper names, or they experience tastes when reading the names of subway stations. How do these rare cases relate to other more common examples where sensory experiences get mixed - cases like mirror-touch, personification, cross-modal mappings, and drug experiences? Are we all more or less synaesthetes, and does this mean that we are all subjects of crossmodal illusions? Could some apparently strange sensory cases give us an insight into how perception works? Recent research on the causes and prevalence of synaesthesia raises new questions regarding the links between these cases, and the unity of the condition. By bringing together contributions from leading cognitive neuroscientists and philosophers, this volume considers for the first time the broader theoretical lessons arising from such cases of sensory blending, with regard to the nature of perception and consciousness, the boundaries between perception, illusion and imagination, and the communicability and sharing of experiences.