Estrogen therapy is a well-known and commonly used symptomatic treatment of hot flashes, night sweats and vaginal dryness associated with menopause. The most active estrogen, estradiol, is also understood to have a protective effect against osteoporosis. Like many hormones, though, estradiol has many roles in the body, both in males and females, and studies of these roles continue to emerge. A major function of estradiol involves obesity, insulin resistance, and cardiovascular disease; studies have shown the beneficial effects of estradiol in these areas, and this is somewhat at variance with traditional belief. In recent years, many researchers have studied its protective, beneficial effect, and have arrived at convincing evidence. In females, and, to some extent in males, estradiol is very important in protecting against obesity and lessening the likelihood of insulin resistance and cardiovascular disease. For both menopausal women and hypogonadal men, estradiol levels should be monitored and considered for long-term healthcare benefits.