This chapter discusses the general factors that have been suggested as underpinning working memory development. The chapter examines in turn the role and impact on working memory development of age-related changes in speed of decay, cognitive resources, processing speed and efficiency, size of the focus of attention, strategy use, and long-term memory knowledge. Change in the speed of decay during childhood is probably the most straightforward idea one can have to account for the increase in working memory capacity. Prominent theories of working memory assume that its main limitation is due to the limited size of the focus of attention. Development of working memory could result from an age-related increase in this size, a conception that echoes neo-Piagetian proposals, especially G. S. Halford's theory. A large part of the working memory literature dedicated to the emergence of strategies focuses on the developmental changes in verbal rehearsal to account for working memory development.