The area of serial memory has had the longest history of research in psychology. It started with Ebbinghaus's interest in relatively permanent memory, evolved into an interest in transfer among lists, and most recently has focused on theories of memory span. It has seen a fair amount of theory in the last third of this century (e.g., Baddeley, 1986; Burgess & Hitch, 1992; Conrad, 1964; Ebbinghaus, 1885; Estes, 1973; Lewandowski & Murdock, 1989; Murdock, 1993; Shiffrin & Cook, 1978; Richman, Staszewski, & Simon, 1995; Wickelgren, 1965; Young, 1968). While the ACT-R theory is applicable to all types of serial recall paradigms, this chapter will describe an ACT-R application to the relatively immediate recall of relatively short lists as this is where most of the recent interest has been. Much of the recent theory has been dominated by Baddeley's theory of the phonological loop. His theory assumes that the amount that can be maintained in a memory span is the number of words that can be rehearsed in approximately 2 seconds. The strong evidence for this proposal comes from research showing that people can maintain fewer words that take longer to articulate-either because the words have more syllables or have syllables that are longer to articulate. In one very influential study Baddeley, Thompson, and Buchanan (1975) looked at the number of words (out of 5) which could be repeated back as a function of syllable length of the words. Varying syllable length from 1 to 5, they found that the amount recalled was approximately equal to the number of words that could be said in two seconds.