Health promotion efforts will play a powerful role as we work to enhance function and reduce morbidity by intervening on modifiable risk factors such as physical activity (PA), inactivity, social engagement, and nutrition. This chapter examines the state of the art of theoretical foundations for health behavior change that are used to design and implement health promotion programs for older adults. The principles of social cognitive theory (SCT) have been used frequently in health behavior interventions. The chapter uses the ecological model as a guide to describe the level(s) targeted by each theory. It presents the most prominent multilevel approach, the social-ecological model. Recently, there has been a push toward broader ways of thinking about behavior change using structural approaches that target all levels of the social-ecological model. The chapter presents theories targeted at each level and argues for the use of multilevel interventions whenever possible.