The main aim of this thesis was to investigate the role of multi-digit number processing in the development of numerical cognition. The most influential contemporary cognitive model of numeracy (Triple-Code model by Dehaene, 1992) differentiates between modality-specific processes (processing of number words vs. Arabic numbers vs. magnitude) as well as between semantic processes like number comparison and asemantic processes like transcoding. Up to date, it has not been tested empirically whether these notions are also adequate to describe the acquisition of multi-digit number processing in primary school children. The findings of this thesis can be seen as clear evidence against the predictions of this model. Furthermore, all tasks assessing multi-digit number processing presented with considerable gender differences in favour of boys. Finally, individual differences in multi-digit number processing were predicted by general visual-spatial abilities.