Delirium is a clinical diagnosis, which is based on observed disturbances of consciousness and cognitive dysfunction. Associated neuropsychiatric and psychomotoric symptoms are common. Many elderly delirious patients are hypoactive, manifesting a passive demeanor, reduced activity and, in extreme cases, stupor and coma. Delirium has an acute onset and a fluctuating course. It is usually reversible. It is crucial to examine cognitive functions among elderly hospitalized patients, since advanced age and preexisting dementia are important risk factors for the development of delirium. In patients with suspected delirium there should follow a further diagnostic approach with specific tests, such as the Confusion Assessment Method (CAM), a diagnostic tool which has a high sensitivity and specifity. Delirium often develops during the course of hospitalization, so repeated screening and cognitive testing is needed. Predisposing factors need to be identified and treated. The cause of delirium is often multifactorial and heterogeneous. Symptoms might be subtle, and clinicians frequently under-recognize delirium, so that it is often diagnosed late in its course. Especially in the elderly delirium can be the only symptom of an acute and severe illness. The typical clinical findings of this illness can be missing. The clinical examination and careful use of diagnostic tools is therefore essential in the identification and treatment of delirium.