The ability to create an accurate, predictive model of the world is one of the most remarkable attributes of the brain. Our goal is to understand how activity and plasticity in neural circuits underlie both the ability to create this model and to use it to make decisions. In this talk I will focus on neural activity patterns that have the potential to play a central role in model-based decision-making. This work began with the realization that the ability to generate and evaluate representations of hypothetical experience, whether of a counterfactual past or of a possible future, has profound adaptive value. How and when the brain might express these representations has not been clear, and I will describe work from my laboratory that has identified and characterized these representations as a surprisingly common motif in hippocampal spiking activity. We have also found that changes in firing rates in medial prefrontal cortex can be seen immediately before and during the expression of these hippocampal representations, consistent with the possibility that this activity marks the time of engagement and evaluation of mental simulations.