The mental number line metaphor describes how numbers are associated with space. These spatial-numerical associations (SNA) are subserved by parietal structures (mainly intraparietal sulcus [IPS] and posterior superior parietal lobule [PSPL]). Generally, it is assumed that this association is a basic cornerstone for arithmetic skills. In this review, we present a taxonomy of SNAs and outline which of them are related to arithmetic skills. Recent research suggests that not all SNAs are related to arithmetic skills; for instance, the spatial-numerical association of response codes (SNARC) is not or at least less related to arithmetic skills than SNAs assessed in the number line estimation task. In general, we conclude that the relationship between SNAs and arithmetic skills are rather weak or caused by mediating variables. Nevertheless, interventions based on relations between space and numbers can be beneficial for arithmetic skills because space is a powerful tool to understand arithmetic concepts.