The hippocampus is composed of several regions, each of which is a network of 10 5 to 10 6 neurons. The pattern of excitatory synaptic connections within and between regions has been determined, and the “wiring diagram” is shown in Figure 1. The intricacies are striking and beg for explanation, but it remains unclear how the connections work together to perform a function. Indeed, more generally, the goal of relating network connectivity to function has not been achieved for any region of the vertebrate central nervous system, with the exception of the retina. For many brain regions, the goal is a distant one, because not even the function of the region is known. For the hippocampus, the goal appears attainable, because the general function of the hippocampus in memory processes is established. Many models that relate aspects of hippocampal circuitry to memory function have have been proposed, and these have become more refined as new information has been learned about the cellular, network, and functional properties of the hippocampus (