Theories of expertise have proposed that superior cognitive performance is in part due to increases in the functional capacity of working memory during domain-related tasks. Consistent with this approach Fincher-Kiefer et al. (1988), found that domain knowledge increased scores on baseball-related reading span tasks. The present studies extended those findings using span tasks with independent storage and processing components, and manipulating which component was related to baseball, to determine the source of functional working memory advantages due to domain knowledge. Only when the storage component was related to baseball, and participants were made aware of that relation, did domain knowledge lead to increases in performance on a domain-related span task. The results are discussed in relation to a Long Term Working Memory explanation of expert performance Ericsson and Kintsch (1995), but also show that the relevance of domain knowledge may need to be explicitly recognized to expand the functional capacity of working memory. (Contains 7 tables and 9 figures.)