Some clinical aspects of the diagnosis, differential diagnosis, and the nosologic classification of the dementia syndrome are presented. In discussing the definition of the dementia syndrome and its differentation from other organic brain syndromes, the criteria of DSM III are taken into account. Considering the nosological classification of the dementia syndrome particular emphasis is placed on the most frequent diseases in the senium, the Multi-Infarct Dementias (MID) and the primary degenerative dementias (PDD, according to DSM III). The clinical problems in differentiating MID from PDD and Morbus Pick from the dementia of the Alzheimer type in the group of are discussed in more detail.