How does the brain encode, store, and retrieve an entire lifetime of experiences? Biological memory capacity has been extensively studied with regard to the short-term retention of recent information (Miller, 1956;Nelson, 2001), and a system with a limited capacity in the single digits has consistently been suggested. Such findings suggest that the brain may have multiple memory systems, and numerous longer term stores (e.g., procedural, episodic, long-term, semantic, associative, and reference memory) have been proposed to capture the extensive knowledge and memories stored beyond those of the recent past. One challenging issue concerns the storage capacity and information retrieval of these longer term memory mechanisms. Attempts to examine the capacity of such longer term memory stores have been limited and mainly computational (Dudai, 1997;Landauer, 1986;Standing, 1973). Although there is general agreement that the human capacity for recognizing pictorial information is considerable (Shepard, 1967;Standing, 1973) and the storage needs for language use substantial, the empirical study of long-term memory capacity has been virtually absent because of the impractical requirements of testing humans over extended periods.Nonhuman animals are similarly guided by a combination of acquired knowledge and recent experiences, but here as well, little effort has been directed at understanding the capacity of their longer term memory systems. Vaughan and Greene (1984) conducted the most extensive study of animal memory capacity. They showed that pigeons could discriminate among 320 randomly assigned pictorial stimuli (160 positive and 160 negative) in a successive go/no-go discrimination. On the basis of a smaller stimulus set (160), they further demonstrated that some of these memories endure for at least 2 years. There are some important limitations, however, to these findings. Their rate-based discrimination procedure and rankorder measure of performance were not designed to yield a measure of how many stimuli were retained, for example. Thus, their pigeons' above-chance discrimination could have been accomplished with considerably fewer than the 320 pictures being memorized. Furthermore, the testing of all 320 stimuli was quite brief, making it impossible to judge the long-term effects of high memory load. Since then, the capacity of pigeons to remember large numbers of pictures has been confirmed. Von Fersen and Delius (1989) used a two-alternative choice proceThis research was supported by a grant from the Animal Behavior program of the National Science Foundation to R.G.C., who thanks everyone in the Cook lab for assistance in conducting these experiments over the last 3 years and Taylor Johnson and Angie Koban for their comments on earlier drafts. Correspondence concerning this article should be addressed to R. G. Cook, Department of Psychology, Tufts University, 530 Bacon Hall, Medford, MA 02155 (e-mail: robert.cook@tufts.edu; home page, www.pigeon.psy.tufts.edu). How much information can a brain store over a li...