Cognition and behaviour are emergent properties of brain systems that seek to maximise complex and adaptive behaviour while minimising energy utilization. Different species reconcile this trade-off in different ways but in humans the outcome is biased towards complex behaviours and hence relatively high energy use. However, even in energy intensive brains, numerous parsimonious processes operate to optimize energy use. We review how this balance manifests in both homeostatic processes and task-associated cognition. We also consider the perturbations and disruptions of metabolism in neurocognitive diseases.