Delirium, the acute confusional syndrome, is a common although infradiagnosed problem in the critically ill patient, especially the hypoactive subtype. Risk factors for delirium are previous cognitive disturbances, some comorbidities, ambiental factors and the acute organic alterations of critical illness. Delirium is associated to an increase in short and long term mortality, prolongation of mechanical ventilation, increased Intensive Care Unit (ICU) and hospital length of stay, and cognitive impairment after hospital discharge. In the last years several tools have been developed to detect delirium in critically ill patients. The Intensive Care Delirium Screening Checklist (ICDSC) and the Confusion Assessment Method for ICU patients (CAM-ICU) have been validated and are useful even in patients receiving mechanical ventilation. Some interventions on specific risk factors can decrease the incidence of delirium in hospitalized patients. Treatment of delirium is based on the identification and correction of contributing factors, the introduction of support measures, and pharmacological therapy for symptomatic control. Halloperidol is the first line therapy of delirium in the critically ill patient, while experience with atypical neuroleptics and other drugs is limited, precluding to do recommendations about its use. Neuroleptic drugs can produce severe side effects and need careful dosage and monitoring. When agitation is important, can be necessary the simultaneous use of benzodiazepines or propofol, and some times, the temporal and protocolized application of physical restraints.