Every year, 1 in 4 people aged 65 and older falls. Twenty-five percent of these falls result in severe injury or death and have devastating effects on quality of life. Despite evidence that physical activities targeting leg strength and balance reduce falls, less than 12% of older adults engage in these activities regularly, and fall rates continue to increase. Research examining intervention strategies that motivate older adults to engage in physical activity is scarce and inconclusive. Thus, there is a compelling need to identify specific behavior change strategies that effectively and efficiently motivate older adults to sustain increased levels of physical activity. In this presentation, I will describe an ongoing 5-year research project, Community-based Intervention Effects on Older Adults' Physical Activity and Falls, that addresses this need. In particular, I will describe how and why we use the Multiphase Optimization Strategy (MOST) framework to guide the methods used in this project.