Many lines of evidence have shown that the interactions between hippocampal and neocortical areas are critical for the process of memory formation. However, the structure of these interactions at the level of networks of neurons remains poorly understood. In this talk I will characterize the organization of theta oscillations across the hippocampal circuitry and the role of these oscillations in the formation of learned associations across prefrontal cortical circuits. Furthermore, I will discuss the consequences of synchronous hippocampal bursts during sleep for the reorganization of hippocampal and neocortical networks.