Delirium is an acute change in mental status, which has the features of a fluctuating mental status, inattention, and either disorganised thinking or altered level of consciousness. It is a common development particularly in hospitalised elderly patients. Delirium is independently associated with adverse sequelae including increased mortality, institutionalisation and cognitive decline. Despite this, case identification remains poor, with up to two-thirds being missed or mis-attributed to dementia, depression and other neuropsychiatric diagnoses. This book discusses how delirium affects the critically ill; early detection of delirium; delirium in surgical patients; and delirium's connection to eating disorders.