A substantial body of literature has suggested that the memory and learning deficits associated with Alzheimer's disease and aging are attributable to degeneration of the cholinergic magnocellular neurons of the nucleus basalis of Meynert (nbM). Subsequently, lesion-induced damage to the cholinergic projections from the nbM to the neocortex has been utilized extensively as an animal model of dementia. In addition, the effect of the normal aging process on deterioration of these neurons and on cognitive function has also been examined. Such studies have revealed, for example, that many of the learning and memory impairments traditionally attributed to the cholinergic corticopetal system are not due to degeneration of the cholinergic neurons of the nbM, but instead may be due to damage of more rostral elements of the cholinergic basal forebrain system. This review will examine the contribution of behavioural animal and human studies to out understanding of the role of the basal forebrain cholinergic neurons in age-related cognitive impairments.