The worldwide demographic explosion in numbers of older people makes the study of cognitive change in old age of obvious practical interest. It is also theoretically crucial because it demands models for cognitive change that can explain how some cognitive skills are retained while others are lost and how tasks that we once performed superbly well are compromised by biological changes in the brain.

A striking weakness of models in contemporary Experimental Cognitive Psychology is that they only describe functional systems that cannot occur in nature because they do not alter with practice, or during childhood development and age-related decline, and are identical in all individuals. Cognitive Gerontologists are deprived of such convenient fictions because their hope is to explain how we gradually become less competent at things that we once did extraordinarily well. They encounter the quite different, and salutary, difficulty that calendar time, the dimension in terms of which old and young people are conventionally differentiated, only indicates the maximum period within which an enormous range of disparate changes can have occurred. These changes certainly include poorly-understood processes of "normal" or "usual" ageing but also the cumulative effects of the pathologies and biological life events that, to varying degrees, affect our brains.

The papers in this special issue, all by leading researchers, highlight the important practical and theoretical advances in the understanding of the impact of these changes in the ageing brain on the ageing of our cognition.