Contrary to prevailing medical belief, dementia is generally not caused by arteriosclerosis of cerebral vessels. There may be, in fact, as many as 50 different causes. While the two major types of dementia, Alzheimer's disease and multi-infarct dementia, are irreversible, perhaps 20 per cent of dementias are secondary to treatable causes, and are reversible. The most important of these is the pseudodementia of depression, which can be easily missed without a high degree of suspicion. Tips on the differential diagnosis of Alzheimer's disease, multi-infarct dementia, and pseudodementia are discussed, as are other causes of dementia secondary to various diseases or therapy.