Although synesthesia has been known about for 200 years, it is only in the past decade or so that substantial progress has been made in studying it empirically and in understanding the mechanisms that give rise to it. The first part of the review considers the characteristics of synesthesia: its elicited nature, automaticity, prevalence, and consistency, and its perceptual and spatial phenomenology. The second part considers the causes of synesthesia both in terms of candidate neural mechanisms and the distal influences that shape this: genetic differences in developmental synesthesia and plasticity following sensory loss in acquired synesthesia. The final part considers developmental synesthesia as an individual difference in cognition and summarizes evidence of its influence on perception, imagery, memory, art/creativity, and numeracy.