The authors examined the nature of the working memory system that underlies age differences of young, preschool-aged children. Measures of working memory, short-term memory, articulation speed, general intelligence, and writing were administered to 166 Canadian preschool-aged children aged 3 to 5 years. Findings generally support the hypothesis that age-related differences in working memory capacity are a function of growth in a general executive as well as processing at lower levels. The results also showed that working memory predicted unique variance in name writing of preschoolers; however, this association was mediated by children's age (experience) and letter-copying skill. (Contains 5 tables and 1 footnote.)