Abstract Cognition and behaviour are emergent properties of brain systems that seek to maximize complex and adaptive behaviour while minimizing 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. Highlights The metabolic costs of goal directed cognition are approximately 5% greater than the ongoing costs of resting neural activity and homeostasis. The brain is organized as a system that minimizes energy consumption while maximizing computation. This principle means that while the brain consumes the largest proportion of energy in the body, it is remarkably energy efficient considering its computational power. Brain energy supply and use is kept in a delicate and dynamic balance, under hormonal control and regulated by transport across the blood brain barrier. Disruption of neural energy homeostasis is associated with cognitive decline and neurodegeneration. Frontier molecular neuroimaging technologies open new opportunities to study brain energy metabolism, with greater spatial and temporal resolution than traditional approaches. https://osf.io/preprints/osf/m5jze
