In this review, the neuropsychological symptoms of different diseases in the elderly are described. After a brief explanation of relevant principles in the neuropsychological assessment of older individuals, a summary of the complex relation between ageing and cognition is presented. It may be concluded that cognitive decline is not an inevitable outcome of ageing, and may well be the result of unrecognised pathology. The term mild cognitive impairment is reserved for patients whose impairment is objectively demonstrable but is not pronounced in more than one domain of cognition and does not seriously affect activities of daily living. The initial phase of Alzheimer's disease is marked by a progressive deterioration of episodic memory. When the process advances, the impairment spreads to other functions, such as semantic memory, language and visuo-spatial ability. Vascular dementia is the second most common type of dementia; however, it is increasingly being recognised that vascular dementia is actually a heterogeneous syndrome and that several vascular pathologies can lead to cognitive deterioration. In contrast to the striking deficits produced by cortical infarcts, lesions of the subcortical white matter are mainly associated with a non-specific slowing of behaviour. Cerebrovascular disease also plays an important role in forms of cognitive decline other than dementia, and as such, it appears to be no less prevalent in old age than Alzheimer's disease. Neuropsychology is an important asset to the study and treatment of cognitive decline, but must be embedded in a multi-disciplinary context.