Adults easily move between spoken number words and written Arabic digit strings. In my presentation I will describe the development of this skill in children from learning single digits to becoming experts in reading and writing multi-digit numbers. I will explore hurdles on this journey and discuss in how far language affects the development of these skills. In the second part I will present results from our longitudinal study showing that number reading and writing skills in Year 1 predict arithmetic development over the next two years as well as early fraction understanding in Year 3. In sum, our results highlight the importance of number transcoding skills in early primary school.