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The purpose of this article is to present a behavioral theory of performance in the simple (classic) memory span experiment. This experiment has been used to investigate hypothetical mental and neurological processes and intelligent function, but behavioral span theory takes the standpoint of event-based constructs. Established empirical limits on memory span are described in terms of span ability,namely, the ability to function under span load.Span load is a count of the number of discriminative stimuli that function concurrently to occasion responding. Span ability is assessed via standard memory span procedures. The theory distinguishes response string from response chain on the basis of the locus of stimulus control; the response string is controlled by the multiple stimuli in a stimulus string or stimulus complex. Response chains are emitted intraverbals. There is no known limit to the length of a chain that can be attained through extensive experiential history, but the memory span experiment assesses the length of response strings that are severely limited at around 7 items (the span limit) and are quite resistant to change. Span limits increase during the developmental period and covary with intelligent function. The values of span ability and span load are potent tools for the prediction and control of developmental and individual differences in diverse experimental paradigms drawn from both behavior analytic and general experimental traditions.