The majority of evidence presented in this paper supports the hypothesis that exercise positively affects bone mineral density in both premenopausal and postmenopausal women. Significantly, exercise has been shown to increase bone mineral density in premenopausal women even after the teenage years, and it helps preserve the bone mass achieved in the following decades. There is also evidence that exercise adds a modest, yet significant amount of bone mass to the postmenopausal skeleton. As these findings demonstrate, women of all ages can benefit by regular weight-bearing exercise, an increased intake of calcium-rich foods, and—for postmenopausal women—the maintenance of adequate estrogen levels. For all women, it is never too late to prevent osteoporosis or lessen its severity by making appropriate lifestyle choices.