The human brain is uniquely powerful with respect to cognitive abilities, yet the hippocampal and neocortical circuits that mediate such complex functions are highly vulnerable to aging. Their selective vulnerability is profoundly manifested in Alzheimer's disease (AD), where degeneration of select neurons leads to a near complete loss of cognitive abilities. Most of us will avoid AD as we age. However, many will experience age-associated cognitive impairment - a decline in cognitive status presenting as deficits in memory and key capacities for strategic use of acquired information. Animal studies suggest that both AD and age-associated cognitive impairment reflect vulnerability of the same circuits. However, neuron death predominates in the former, whereas the latter is probably mediated by synaptic alterations in otherwise intact circuits. Fortunately, such age-related synaptic alterations could be reversible, as suggested by recent studies of hormone replacement.