Abstract
Number-relevant information bombards our everyday lives. It is necessary for telling the time, for dealing with money, for changing TV channels, for shopping, for taking local transport, and so forth. Human children show sensitivity to numerically relevant displays very early in infancy and continue to develop number processing skills and knowledge about number facts throughout childhood. Yet, despite dyscalculia being more prevalent than dyslexia, particularly in children with neurodevelopmental disorders, studies of atypical number development are far less plentiful than those of atypical reading. This chapter examines the neural and cognitive underpinnings of typical and atypical numeracy, in particular numerical cognition in infants, children, and adults with Williams syndrome (WS), but also referring to other neurodevelopmental disorders, with particular emphasis on the distinction between numerical sensitivities vis-é-vis small numbers and approximate magnitude computations with large numbers.