Park and recreation settings are viable locations for promoting health enhancing physical activity. Investigations of physical activity and associated variables in “open” environments have been hampered by lack of objective tools. Measuring activity in these environments is challenging because the number and type of users and their activity levels are highly variable and park characteristics change often. This paper describes the evaluation of the observation instrument, System for Observing Play and Recreation in Communities (SOPARC). SOPARC is based on momentary time sampling techniques in which systematic and periodic scans are made of individuals and contextual factors within pre-determined target areas in community recreation settings. During a scan the activity of each individual is coded as Sedentary (lying down, sitting, or standing), Walking, or Very Active using specialized counters. Separate scans are made for females and males, and simultaneous entries are made for user age and race/ethnicity and park characteristics (e.g., time of day; area accessibility and usability; presence of supervision and equipment; presence and classification of organized activities). Reliability data were collected during 16 days of field assessment over 4 months in 4 large parks (total=85 different activity areas) by pairs of trained assessors who made simultaneous and independent observations throughout the day. Data from a total of 335 simultaneous measures were used in the reliability analysis. Interobserver agreement scores (IOA) for contextual variables were 95% for area accessibility, 91% for usability, 95% for presence of supervision, 95% for presence of organized activity, and 96% for provision of equipment. Intraclass correlations for the number of park users were high (R=.99 for females; R=.97 for males). IOAs for age grouping (89% for females; 85% for males), race/ethnic grouping (80% for females; 82% for males), and activity levels (80% for females; 88% for males) also met acceptable criteria for reliable assessment. The utility and generalizability of SOPARC were established through the use of the instrument to generate data during morning, noon, afternoon, and evening observations in large parks (170 targeted activity areas) in 8 multi-ethnic communities during 56 days. Because validity of the activity codes has been established through heart rate monitoring, energy expenditure rates for target areas can be estimated. The instrument permits physical activity level and other comparisons to be made among different settings or within the same environment over different time periods (e.g., for intervention evaluation purposes).Keyword(s): assessment, community-based programs, physical activity