Because of better medical care and improved general social conditions, the population over age 65 is increasing rapidly. It is estimated that this population will comprise about 20% of the U.S. population by 2030 (Friis, Nomura, & Swan, 2003). To maintain older adults' health and quality of life, participating in regular physical activity is crucial, since studies have shown clearly that participation in physical activity could reduce older adults' risk of cardiac diseases, delay mortality and enhance longevity. Walking is perhaps the best physical activity/exercise format for older adults since it is inexpensive, easy to participate in, and has less risk of injury (Friis et al., 2003). However, only a very small proportion of older adults participate in physical activity, including walking regularly. Many factors are believed to limit participation in physical activity among older adults, e.g., poor health and lack of appropriate or convenient exercise facilities (AARPRSG & AOA, 2003; Cardinal & Spaziani, 2003). In addition, the lack of a safe and supportive environment is believed to be another important determinant. Very few studies so far have been conducted to identify determinants that may directly impact the walking behavior of older adults. Measurements employed in a few available studies of walking (e.g., pedometers) measure only the walking behavior itself; not its interactions with environment and determinants. Based on the concept of physical activity space, a study has been designed to study older adults' walking space. Using a combination of GPS and step measures, older adults' walking behaviors were measured and, for the first time, detailed walking information, such as when, where, how many steps and what kinds of steps (e.g., slow vs. fast), were collected and studied. This presentation will provide a detailed description on the walking behavior of older adults. Using this framework of physical activity space, walking space of older adults will be examined and described. Challenges in measuring walking space will be emphasized and future research directions will be described. Keyword(s): measurement/evaluation, older adult/aging issues, physical activity