The hippocampus, long recognized as important for episodic memory function, is increasingly also seen as part of a network that is important for the imagination of future episodes. By recording from many hundreds of neurons simultaneously in freely moving animals, we show that hippocampal neurons are activated in sequences corresponding to temporally-compressed spatial episodes, in a phenomenon often termed "replay". These sequences occur frequently during the awake state, and I will present several features of this activity in novel environments that suggest that learning is required, and that extra-hippocampal circuits process this information. I will also show that, in a spatial memory task, the sequences can proceed from the animal's current location to a remembered goal location, and depict the route that the animal is about to take, even when the specific combination of start and end locations has not been experienced before, i.e. when the trajectory is novel and must be inferred from the animal's prior experience. I will further present evidence from animal models of psychiatric disease showing that these sequence events are specifically impacted. Together, these results suggest a neuronal model system for understanding high-level cognitive function in both normal and disease states.