Extant research findings allow several conclusions regarding the relationship between estrogen and cognitive functioning across the female life span. First, performance on tests of verbal memory fluctuates in concert with physiological changes in ovarian hormone production during the menstrual cycle and during pregnancy and the postpartum period. Estrogen therapy (ET) prevents the decrease in verbal memory when administered immediately following the surgical removal of both ovaries in premenopausal women. Some, but relatively little evidence is available to support the idea that ET, initiated at the time of a natural or a surgical menopause for a few years, may protect against cognitive decline 30 years later and more research in this area is urgently needed. Finally, the evidence to date strongly suggests that the initiation of ET decades after the menopause has occurred does not protect against cognitive decline or dementia. Taken together, these findings support the so-called "window of opportunity" hypothesis which holds that ET will be neuroprotective only when administered closely in time to a natural or surgical menopause.