Delirium is a common term for an ancient condition that still defies precise categorization or treatments based explicitly on its pathophysiology. Understanding has proved evanescent. For example, even a tried-and-true distinction between delirium and dementia appears not to withstand precise scrutiny. A newer understanding of the pathophysiology of delirium focuses on the role of neuroinflammation in both engendering vulnerability in the ageing and/or cognitively impaired brain and, when unwell with systemic disease, precipitating delirium. In the meantime, useful effort has focused on prevention, especially by multicomponent interventions. Treatment proceeds by addressing any precipitating illnesses and by general measures, including empathetic, personalized care. A role exists for dopamine antagonist medication, but it must be used with a precise indication and with early and frequent review. The prognosis of delirium is now understood to be often less transient than had originally been proposed, especially when it occurs in older people, in many of whom it can unmask, or even precipitate, dementia. Future research should extend to elderly people, who, in any setting, develop delirium, and evaluate mechanisms that lead to the characteristics features of delirium.