How do we decide what is the best course of action? How do we use prior experience to guide these actions? How do we decide when outcomes are delayed and uncertain? How do we deploy a model of our environment to assist us in choosing? This is the Ferrier prize lecture given by Professor Ray Dolan FMedSci FRS, who was awarded the Ferrier Medal and Lecture 2019. Ray Dolan’s group, and collaborators, have tackled such questions over the past two decades. His group uses state of the art brain imaging techniques, combined with a mathematical analysis of behaviour, to probe how these abilities arise out of brain function. This research has revealed new and exciting principles of brain organisation and has also provided insights into how processes related to planning go awry in severe forms of psychiatric disorder. The Royal Society is a Fellowship of many of the world's most eminent scientists and is the oldest scientific academy in continuous existence. Visit our website to learn more: https://royalsociety.org/ The Royal Society publishes leading science journals. Stay informed: https://royalsociety.org/journals/