Delirium is a neuropsychiatric syndrome often manifesting in acute disease conditions, and with a greater prevalence in the older generation. Delirium in the Emergency Department (ED) is a highly prevalent problem that typically goes unnoticed by healthcare providers. The onset of a delirium episode in the ED is associated with an increase in morbidity and mortality. Because delirium is a preventable syndrome, these statistics are unacceptable. Emergency Department staff therefore should strive to perform systematic screening in order to detect delirium. Different tools have been developed for the assessment of delirium by healthcare professionals other than psychiatrists or geriatricians. Emergency Departments require delirium assessment scales of high sensitivity and specificity, suited to the characteristics of the Department, since the time available is scarce. In addition, the presence of dementia in the assessment of delirium may induce sensitivity bias. Despite the existence of numerous delirium rating scales, scales taking less than three minutes to complete are recommended. The choice of the tool depends on the characteristics of the ED. The only scale affording high sensitivity and specificity in older people with and without dementia is the Four “A”s Test (4AT); it requires no training on the part of the rater, and can be performed in under two minutes.