There is increasing evidence that exercise promotes psychological and physiological benefits for older individuals. Nonetheless, the prevalence of sedentary lifestyles increases, as people get older. Given the aging of the population, a major challenge for health professionals is to develop and deliver programs that enhance and maintain health status, and thereby, quality of life. The proposed randomized controlled trial takes a social cognitive approach (Bandura, 1977, 1986) to experimentally evaluate an intervention designed to improve physical and psychological control and subsequently, health status, using a moderate-intensity, low-cost, Tai Chi program for a vulnerable population of older adults. A four-year study is proposed with the primary objective of understanding the long-term effectiveness of Tai Chi in: (a) promoting psycho-social and physical benefits; (b) improving health status and quality of life, and (c) influencing known determinants of continued physical activity participation. Using a randomized control intervention trial, the study will focus on the effects of a Tai Chi program on theory-based mediators of changes in a variety of psychological control variables including self-efficacy and physical control variables including balance, among older individuals aged 70+ years, by comparing the overall difference in change between intervention (n=112) and exercise control (n=112) groups over time. The results are expected to provide a better understanding of the effects of Tai Chi on the targeted psychosocial and physical characteristics of older individuals, its sustained effects on their health, quality of life and exercise participation, and the feasibility of such programs for relatively low cost implementation in community facilities.