Research on working memory has revealed a developmental trend in memory performance, but a controversy exists as to whether the improved performance is due to a developmental increase in working memory total processing space (M-space), is the result of more efficient processing and storage of material, or is due to the use of "chunking" or other mnemonic strategies by older children. A study traced the development of working memory span and examined the relationship between metacognitive knowledge about reading and memory, M-space, and working memory performance in skilled and less-skilled readers. Subjects, 32 second, fourth, and sixth grade children, completed a series of memory and metacognition measures and M-space tasks. Results substantiated a developmental increase in working memory span performance. The pattern of grade and reader effects found for the span tasks suggests that the trend in working memory performance resulted from an increase in the processing efficiency of mental operations, rather than from an increase in the total processing space. Metacognitive ability was marginally related to working memory performance. (FL)