The aetiology of the common dementias of old age remains incompletely understood. Here we describe some of the biological, neurophysiological and psychological changes associated with ageing of the human brain, in terms of those that occur throughout life and those that are characteristic of senescence. Age-dependent diseases, such as Alzheimer's disease (AD), idiopathic Parkinson's disease (IPD) and dementia with Lewy bodies (DLB), are considered from these viewpoints, and risk factors described. Vascular dementia (VaD) is related to hypertension and atherosclerosis and detailed description of its pathogenesis is outside the scope of this review. The importance of age as the main risk factor raises basic questions about the relationship of these diseases to the ageing process itself. Similarities and differences between ageing and disease may be important for a rational approach to prevention and treatment of cognitive decline and dementia in later life.