The ability to understand and manipulate numerical symbols (Arabic digits) is at the heart of everyday numeracy. However, numerical symbols are cultural inventions and it is still debated how children acquire the meaning of numerical symbols. In my talk I will focus on children’s knowledge of Arabic digits at the beginning of primary school. In the first part of the talk I’ll explore mirror writing of digits in children in early primary school and its relationship to mathematical development. While children in early primary school still frequently mirror write letters and digits, other aspects of their number writing are actually more predictive of their later mathematical development. In the second part of my talk I will focus on those aspects and present data suggesting that children’s ability to transcode accurately between spoken number words and written Arabic digits for multi-digit numbers at the beginning of primary school is a good predictor of their arithmetic growth in early primary school. Our findings show clearly that the ease with which children acquire the meaning and rules of numerical symbols needs to be considered in theories of individual differences in, and the development of, mathematical skills.