Abstract People with synesthesia have a remarkable way of experiencing the world in which numbers may evoke colors, words may trigger tastes, or the silent movement of an object may elicit a sound. In this chapter, we present an overview of the different kinds of synesthesia and the challenges faced in proposing a common explanation (and definition) of such a diverse collection of experiences. We focus specifically on developmental synesthesia, summarizing evidence that the brains of synesthetes have certain functional and structural differences, and discuss theoretical accounts as to how synesthesia might emerge developmentally. These accounts include both a role of a genetic component and environmental influences, where the latter seem to be particularly important for shaping the particular associations. Finally, synesthesia is linked to a distinct pattern of cognitive abilities affecting imagery, memory, and perception.