Statistics indicate that in Europe people over 65 years of age will be 23.5% at the beginning of the next century and prevalence of severe dementias is estimated to be 5% in this population group. Clinically, dementia is a syndrome characterized by memory loss, other cognitive dysfunctions and loss of ability for self-care. This organic syndrome, that can have many different causes, must be differentiated from other similar psychiatric disorders of the elderly such as the pseudo-dementias. The most important of these causes are Alzheimer's disease (AD) and vascular dementias. Other causes are infections, toxic and metabolic disorders, normal pressure hydrocephalus and head trauma. Many efforts are required in order to reach an accurate differential diagnosis, as 10-15% of the dementia syndromes are reversible if an appropriate therapy is applied. Because of its high frequency, AD is one of the most actively studied areas in dementia research. The specific causes of AD are still unknown, but recent case-control studies suggest the importance of risk factors such as familiarity and previous head trauma. Other hypotheses concern prion infections, aluminium toxicity and immunologic disorders. The most important findings of the last years however, concern biochemical alterations in Alzheimer's brains. After the first observations of Davies and Maloney (1976), who observed a reduction of choline acetyltransferase in cerebral cortex, many reports successively indicated the importance of deficiencies of the cholinergic systems in AD. Other neurotransmitter systems, such as the noradrenergic and the serotoninergic systems, were also found to be involved in AD. From these findings a rational therapeutic approach to the disease was proposed. Initially, the clinical trials employed physostigmin with uncertain results. Later, therapeutic attempts with choline, lecitine, acetylcarnitine and phosphatidilserine resulted more promising, at least in the initial phase. However, consistent data are not yet completely available. Finally, the management of AD also concerns problems of familiar education to the behavioural management of AD patients and the eventual possibilities of social assistance. Vascular dementia and Creutzfeldt-Jakob disease will also be discussed.