This study investigated whether processing speed, short-term memory, and working memory accounted for the differential mental addition fluency between children typically achieving in arithmetic (TA) and children at-risk for failure in arithmetic (AR). Further, we drew attention to fluency differences in simple (e.g., 5 + 3) and complex (e.g., 16 + 7) mental addition. Results suggested two important findings. First, working memory completely accounted for simple mental addition fluency differences between TA and AR children. Second, while working memory had the strongest effect on reducing differences in complex mental addition fluency between TA and AR children, group differences remained after accounting for the contributions of processing speed, short-term memory, and working memory. Results are discussed in terms of directions for future research on the cognitive processes that contribute to mental addition fluency. (Contains 5 tables and 1 figure.)