What is the nature of the relationship between different lower-level numerical skills and their role in developing arithmetic skills? We consider the hypothesis of a reciprocal relationship between the development of symbolic (e.g., Arabic numerals) and nonsymbolic (e.g., arrays of objects) numerical magnitude processing. Evidence for bidirectional relationships between symbolic and nonsymbolic numerical magnitude skill development is examined. Overall, present evidence is more indicative of an influence of symbolic numerical magnitude skills on the development of nonsymbolic numerical magnitude skills than vice versa. Looking forward, methodological issues pertinent to measuring the direction of such relationships are discussed. Also discussed is the important role that training studies need to play to further understand the complex relationships between basic number skills, and in turn their relationship with arithmetic. It is important that assumptions about relationships between lower and higher-level cognitive skills are tested empirically and that seemingly counterintuitive relationships are given serious consideration.