A general orientation toward the study of skills and their development is outlined, in which analyses of representation, transfer, and context are used to explore the consequences of developing a specific skill. This general approach is then applied to the study of abacus training and its implications for school achievement and cognitive development. Previous studies of abacus skill are reviewed, and two new studies are reported. Previous research showed abacus training to result in qualitative changes in children's representations of mental calculation through development of a "mental abacus." In Study 1, mental abacus skill was found to develop primarily as a result of practice rather than of selection factors such as socioeconomic status, ability, and previous mathematical knowledge. Abacus skill did, however, have positive effects on future achievement. Study 2 clarified the mechanisms underlying the functional relationships discovered in Study 1. Abacus training was found to affect both calculation skills and conceptual knowledge of the numeration system. The consequences of abacus training are both varied and limited. Consideration of effects of abacus training on the mental representation of mathematical knowledge, transfer of abacus skill to other tasks, and the contexts in which abacus skill develops demonstrates the multiple ways in which specific skills can contribute to cognitive development.