When interacting with complex environments, humans can rapidly adapt their behaviour in response to changes in task or context. To facilitate this adaptation, people often spend substantial periods of time contemplating possible futures before acting. In this talk, I will present empirical and modelling work exploring the critical balance between thinking and acting, and the factors affecting the content of our thoughts when we are making a decision. I will describe a neural network model that learns to plan when planning is beneficial. This model explains variations in human thinking times and accounts for neural activity recorded from the rodent hippocampus during navigation tasks. This work integrates neuroscience, psychology, and computational modelling to shed light on the neural basis of flexible decision-making.