In this review, studies on number processing and calculation in demented patients are summarized. Single cases, group studies and our own examinations all show that acalculia is a frequent symptom of dementia and correlates with severity of dementia. While calculation and numeral transcoding deficits are often prominent in early courses of the disease, deficits in semantic processing and basic number processing are less severe. Still, single cases demonstrate that very specific dysfunctions in number processing and calculation can occur in dementia. In number transcoding, a characteristic error type occurs in demented patients that has not (or rarely) been described in other brain damaged people so far. In these "shift errors" or "intrusion errors" the wrong number code is used or the number codes are mixed (e.g. patients write "2hindert11" instead of zweihundertelf (two hundred eleven)). Due to the frequent occurence of acalculia in dementia and its specific features, it is suggested that acalculia should be integrated into the criteria for dementia and that number processing and calculation tests are used in the diagnosis of dementia.