An extensive literature suggests a role for the prefrontal cortex and hippocampus in episodic memory, particularly in the re-construction of those memories; a growing literature is now suggesting a role for these structures in planning and imagination, particularly in the construction of future potential episodes, a process termed ``episodic future thinking''. These two processes have obvious similarities (they both entail construction of episodic representations) and obvious differences (memory is trying to reconstruct a real past, imagination is trying to hypothesize a potential future). However, it is not known how this neural circuit contributes to these phenomena, nor how the information processing differs between them. New mathematical techniques now allow decoding of represented variables from neural ensembles at very fast timescales, enabling the observation of transient, covert processes. The goal of this proposal is to use these new techniques to track changes in representation at cognitive (10 ms) timescales. Combining newly available multi-structure recording techniques, newly developed tasks, newly improved neural ensemble analysis techniques, and causal manipulations of the prelimbic cortex, we will examine the relationship between the representations of past events and future possibilities in hippocampus to determine the mechanism by which the hippocampus computes these representations.