The landmark women's health initiative memory study (WHIMS) program has had an enormous impact on understanding of how estrogens and estrogen-containing hormone therapy affect cognitive outcomes in postmenopausal women. This chapter focuses on the impact of hormone therapy on cognition in older women. It summarizes the primary findings of the women's health initiative (WHI) on cardiovascular disease. The WHIMS also found a small adverse effect of assignment to hormone therapy on global cognitive function, as measured by modified mini-mental state (3MS) examinations. The Women's Health Initiative Study of cognitive aging (WHISCA) was designed to examine the impact of conjugated equine estrogens (CEE)-based therapy on longitudinal changes of cognitive tests targeting several different domains. The women's health initiative memory study of magnetic resonance imaging (WHIMS-MRI) was designed to contrast neuroradiologic outcomes among women who had been assigned to active versus placebo therapy during the WHIMS trials.