As a result of both longer life expectancy and declining birth rates, in almost every developed country the number of people aged over 60 years is growing faster than the number in any other age group. This demographic shift is causing profound social and economic challenges that highlight the need to further understand the factors that affect health and quality of life in our later years.

In this issue of Nature Reviews Neuroscience, we launch an article series on 'Brain ageing' that aims to bring together the latest findings on the changes in structural and functional plasticity that characterize healthy brain ageing, the emerging mechanisms that underlie age-related changes in the brain, and the factors that influence them.

Many older adults have a reduced ability to learn and retrieve both verbal and non-verbal material and to recall real-life events. Substantial age-related decreases in attention, spatial memory and some perceptual processes have also been reported. Indeed, a small decline in cognitive performance occurs in the absence of distinct pathology in nearly 40% of people aged 60–78 years. It is now clear that neuronal loss does not markedly contribute to age-related cognitive impairments and that the changes that occur during normal ageing are more subtle and selective than they were once thought to be. Importantly, these changes are not irreversible: studies have shown that age-related differences in cognitive ability can be reduced or eliminated, highlighting the possibility of restoring cognitive function in aged individuals.

Successful ageing is partly determined by genetic background, but factors associated with lifestyle and culture also have an effect. CNS ageing may also be secondary to ageing processes in other organ systems, and there is evidence that the body's hormonal status influences neural and cognitive ageing. This suggests that brain ageing must be viewed from a broad physiological perspective rather than as a process that is isolated from other organ systems. Finally, it is worth noting that the ageing process is remarkably conserved across species, and both cross-species comparisons and studies with exceptionally long-lived organisms are likely to shed new light on human brain ageing.

We begin the series with a Science and Society article by Stranahan and Mattson that explores the neuroprotective mechanisms underlying the beneficial effects of exercise and of specific dietary components, and the possibility of harnessing these mechanisms to protect against the deleterious consequences of brain ageing.