A 3-week problem-solving practice phase was used to investigate concept-procedure interactions in children's addition and subtraction. A total of 72 7- and 8-year-olds completed a pretest and posttest in which their accuracy and procedures on randomly ordered problems were recorded along with their reports of using concept-based relations in problem solving and their conceptual explanations. The results revealed that conceptual sequencing of practice problems enhances children's ability to extend their procedural learning to new unpracticed problems. They also showed that well-structured procedural practice leads to improvement in children's ability to verbalize key concepts. Moreover, children's conceptual advances were predicted by their initial procedural skills. These results support an iterative account of the development of basic concepts and key skills in children's addition and subtraction. (Contains 3 figures and 6 tables.)