Up to 33% of all elders and 40% of elders over age 70 experience knee osteoarthritis (OA), a leading cause of pain and disability. Further, up to 15.3% of elders age 65 have CI, and the prevalence of cognitive impairment (CI) doubles every 5 years after age 65. The prevalence of OA in elders with CI is comparable to that in elders without CI. Cognitive impairment limits elders' ability to perform daily activities, and their functional capacity declines more rapidly than in elders without CI. Having knee OA pain in addition to CI further limits elders' activities. Without proper treatment of knee OA pain, elders with CI may avoid basic daily activities, such as rising, walking, standing, and climbing stairs because these aggravate pain. By avoiding these basic activities, they gradually lose muscle strength, range of motion, and mobility, which leads to further physical deconditioning and social isolation. With aging of the baby boomers and advances in health care, the number of elders with both CI and OA will increase fourfold by 2050. Alleviating knee pain in elders with CI and knee OA could preserve their functioning, perhaps delay institutionalization, and save healthcare dollars. Since pharmacological interventions produce serious side effects and inadequately reduce pain, especially in elders with CI, adjuncts such as Tai Chi (TC) are needed. A low-impact aerobic exercise, TC involves slowly stretching the limbs and trunk and ultimately re-establishes normal mechanics of the knee joints, which reduces knee OA pain. The United States Arthritis Foundation and the American Geriatrics Society have endorsed TC to reduce knee OA pain; but no study has investigated the effect of TC on knee OA pain in elders with CI. The primary aims of this study are: