This paper is concerned with the role of meaning and understanding in the acquisition of computational skill in subtraction. Evidence on the syntactic nature of subtraction errors is reviewed, with common "bugs" described. Then, data collected from interviews with four children are summarized in terms of their semantics of concrete representations of the base system, procedures for adding and subtracting with concrete representations, semantics of the writing code, procedures for written subtraction and addition, and mapping. Finally, linking syntax and semantics and why mapping works are discussed. (MNS)