Delirium is very common in the elderly. It complicates both psychiatric and somatic disorders and is associated with reduced survival, poor functional results, increased duration of hospital stay, and institutionalization. Diagnosis remains difficult in spite of the improvement of the diagnostic criteria, due to the polymorphism of the clinical signs and fluctuation of vigilance and cognition. Age over 70 and previous cognitive impairment are the main risk factors. Precipitating factors are medical and surgical pathologies, intoxications, especially by therapeutic drugs. Delirium can reveal or complicate a previous dementia. Prevention of delirium and care of the delirious patient require the participation of both the medical and nursing staff.