The hypothesis that the physical substrate of memory in the mammalian brain resides in alterations of synaptic efficacy has been proposed frequently in both neuroscience^1–5 and cognitive science^6–12, and has been widely investigated in behavioural, physiological and theoretical studies. Although this hypothesis remains unproven, considerable evidence suggests that a particular form of synaptic strengthening, induced by electrical stimulation of certain CNS fibre systems, may represent the activation of mechanisms that normally subserve associative memory. This phenomenon is known as long-term potentiation (LTP) or long-term enhancement (LTE)^∗. It has been most intensively investigated within the hippocampal formation, a brain structure that plays a crucial role in certain forms of associative memory. Physiological investigation has revealed that LTE exhibits most of the properties implicit in Hebb's original suggestion that associative memory results from a synaptic strengthening that is contingent upon the conjunction of activity in pre- and post-synaptic elements. In this article, we outline a simple neuronal model capable of superimposing multiple memory traces within the same matrix of connections, and consider the correspondence between such models and the properties of LTE in the context of the hippocampal circuitry in which it occurs. Certain predictions are derived from this framework concerning the behavioural consequences of experimental manipulation of LTE, and we conclude by describing experimental evidence that confirms these predictions and suggests that LTE is, in fact, fundamentally involved in memory.