Introduction Ever since the concept of working memory was fi rst described by John Locke (1690) as “contemplation” (see discussion in Logie, 1996), the understanding of temporary memory and on-line cognition has undergone a range of transformations. The Edinburgh philosopher David Hume (1748/1955) described it as “perceptions of the mind”; William James (1890/1902) alluded to the “specious present” or primary memory; George Stout (1898) described the concept of ideation, while Frederick Bartlett (1958) explored it as the vehicle for thinking. More contemporary theoreticians have referred to short-term memory (Atkinson & Shiffrin, 1968), working memory (Baddeley, 1986, 2007; Baddeley & Hitch, 1974; Osaka, Logie, & D’Esposito, 2007), and links with consciousness (Baars, 1997; Logie, 1995, 2009). With the change of name and increased research activity on the topic has come increased complexity and increased debate, along with use of the same term to refer to related but different concepts. In this chapter we focus on one contemporary approach to the study of working memory that considers primarily the impact of differences between individuals in their working memory capacity, and what might underlie those differences. We will contrast two different theoretical perspectives, one of which considers working memory as an ensemble of domain-specifi c functions, the other of which views working memory primarily as activated long-term memory coupled with controlled attention. We note that some differences are more apparent than real; others echo longstanding debates thought to be resolved by previous generations of researchers; whereas key contrasts arise simply because researchers are asking fundamentally different questions.