Delirium is an acute change in mental status, which has the features of a fluctuating mental status, inattention, and either disorganized thinking or altered level of consciousness. It is a common development particularly in hospitalized elderly patients. Delirium is independently associated with adverse sequelae including increased mortality, institutionalization and cognitive decline. Despite this, case identification remains poor, with up to two-thirds being missed or misattributed 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.