Delirium in critically ill patients is a common entity in the intensive care unit (ICU) and is an expression of the cerebral organ dysfunction of the patient. The hallmark signs are disturbed consciousness and cognition in combination with inattentiveness and alterations in perception, which are manifested within a time interval of hours to days during treatment on the ICU. Delirium has been shown to have negative effects on patient short-term and long-term outcome parameters and increases morbidity and mortality. Despite its significance in many cases delirium remains inadequately diagnosed during routine treatment by ICU personnel. There are two validated and easily applicable scales for the standardized diagnosis of delirium: the confusion assessment method for the ICU (CAM-ICU) and the intensive care delirium screening checklist (ICDSC). These are simple to apply by medical as well as non-medical personnel. The therapy of delirium is mostly determined by non-pharmacological measures aiming at early identification, reorientation and mobilization of the patient, improving cerebral activity and establishing adequate wake-sleep cycles. There is only sparse evidence for pharmacological treatment of delirium; however, the choice of sedative agent has a proven effect on the incidence and duration of delirium in the ICU.