Because studies and interventions based only on psychological theories/models have not produced impressive or long-term increases in physical activity on a large scale (Dishman & Sallis, 1994), researchers began to explore other, more effective, models and determinants. The ecological model, which studies the influence of the surrounding environment on individuals' behaviors, has recently earned significant attention. To fully understand the impact of the environment on individuals' physical activity behavior, new concepts and measures are needed. This is because existing physical activity measures measure only physical activities themselves, but not their environment and related interactions. To address research and measurement needs, Zhu (2003) proposed a new concept called "physical activity space (PAS)" based on research literature in human geography. PAS is defined as "the area, or space, within which an individual spends time and engages in physical activities." There are three major components in an individual's PAS: (a) movement within or near the home/residence site; (b) movement to and from regular activity locations, such as travel to work, to shop, to socialize, or to exercise; and (c) movement in and around the locations where those activities occur. Time, space, and corresponding activities are the key elements in measuring physical activity space. In the past, measuring time, space and activity simultaneously has been a challenge. Fortunately, with recently developed satellite technology-based location devices, it has become convenient to capture an individual's physical movements through space and time using the Global Positioning System (GPS). Combining the GPS information collected with Geographic Information System (GIS) datasets, a new way to potentially study individuals' physical activity patterns and environmental interactions becomes possible. Furthermore, with a combination of PAS, existing psychological models and other individual determinants, multi-level modeling of determinants of people's physical activity behavior becomes possible. There are, however, many challenges in measuring PAS. For example, although PAS can now be measured using separate physical activity and space measurement devices, a single-unit device that can collect all of the information simultaneously is greatly needed. Understanding and developing appropriate spatial statistics should also be a priority so that collected information can be examined effectively. Finally, there is an urgent need to develop low-cost and practical measures of PAS so that large-scale studies or interventions can be implemented. This presentation provides an introduction to the concept of PAS, its measures and addresses related issues and challenges.Keyword(s): community-based programs, measurement/evaluation, physical activity