Researchers at the Harvard Prevention Research Center at Harvard University School of Public Health examined how physical and social environments of schools and neighborhoods shape routine physical activities of students attending 10 middle schools in the Boston area. They also analyzed the effect of weather conditions on student physical activity. Key findings include: (1) Larger school campuses, school buildings and play areas per enrolled student were associated with increased physical activity during the school day; (2) Higher temperature on the weekends is associated with increased physical activity on the weekends; and (3) Higher density of land use in neighborhoods around schools is associated with increased physical activity on weekends. Density of use is the number and mix of resources that students value, such as stores, libraries or service organizations. A bibliography is included.