Recent research has highlighted the importance of both Arabic digit knowledge and the early use of exact number words for mathematical development. Once the mapping between number words and Arabic digits is acquired, humans can easily switch between these two symbolic codes. However, there is evidence that number word characteristics continue to influence Arabic number processing well into adulthood. In this talk I will present the results of a longitudinal study in primary school children that shows that the mapping from number words to Arabic digits is an important longitudinal predictor of their arithmetical development. In the second part of the talk I will discuss several studies on children and adults investigating the effect of number word structure on Arabic digit processing/writing.