The term delirium refers to a serious change in mental status resulting in confused thinking and reduced awareness of the environment. In addition, delirium may be accompanied by perceptual symptoms, such as hallucinations, or cognitive symptoms including disorientation or memory dysfunction. Patients with delirium may express hyperactive or hypoactive psychomotor behaviors. The former is expressed with agitation or aggressive behaviors, while the latter with inactivity and drowsiness. Sometimes there is a combination of both types of clinical manifestations. Delirium can often be triggered by several conditions, such as infection, medications, electrolyte imbalance, immobilization, or alcohol or drug withdrawal. A special consideration is postoperative delirium defined as delirium that happens after surgery. Delirium is the most common post-operative complication in older adults and represents a medical emergency that requires immediate assessment and treatment because it has been shown to be associated with a longer and more costly hospital stay as well as with increased postoperative morbidity and mortality. Delirium may also affect the pediatric population at the emergence from general anesthesia and manifesting itself as a status of psychomotor agitation. However, this condition may differ in presentation from the well-defined postoperative delirium which occurs especially in the elderly, after a relatively normal initial postoperative course, generally after the initial 24 hour postoperative period. Postoperative delirium must also be distinguished from postoperative cognitive dysfunction, a decline in cognition abilities temporally associated with surgery but not attributable to delirium per se. This chapter addresses the pathophysiology, the features, and the assessment of postoperative delirium, the strategies useful for the prevention of this serious complication, and its pharmacological and non-pharmacological management.