Understanding the impact of neighborhood characteristics of older adults’ walking behavior and finding ways to facilitate more physical activity into a healthy lifestyle are fast becoming a national health imperative. This study will examine the influence of characteristics of older adults’ environment on their walking behavior. A new physical activity space (PAS) measure and a new simulation method will be used in the study. Older adults in 2 neighborhoods in Champaign and Urbana, Illinois will be recruited and their physical activity space will be measured for 7 days. The particular characteristics that influence walkability within their physical activity space will be studied using a survey developed for the study. Understanding walkablility issues for older adults will have a profound effect on health and health care, given the current obesity epidemic and a rapidly increasing older population.