As indicated by the distance effect and the spatial-numerical association of response codes (SNARC) effect, natural numbers are mentally represented on a number line. Purportedly, this number line underlies children's number sense, which supports the acquisition of more advanced mathematical competencies. In 3 studies with a total of 429 fifth and sixth graders, the authors compared the influences of each child's distance effect, SNARC effect, conceptual knowledge about decimal fractions, and numerical intelligence on mathematical school achievement. Additionally, they tested using decimal fractions whether number line estimation competence mediates the influence of the internal number line. In all, the results, found with path models, revealed that domain-specific conceptual knowledge, numerical intelligence, and number line estimation each were good predictors of achievement, while distance and SNARC effects were virtually unrelated to all other variables. Individual differences in the use of the internal number line, as assessed by these 2 effects, seem to be of little importance when it comes to the acquisition of the content of 5th- and 6th-grade mathematics lessons. The results instead highlight the importance of conceptual understanding and estimation competence. (Contains 9 tables and 3 figures.)