The System for Observing Play and Leisure Activity in Youth (SOPLAY) is based on momentary time sampling techniques in which systematic and periodic scans of individuals and contextual factors within pre-determined target areas are made. During a scan the activity of each individual is mechanically or electronically coded as Sedentary (lying down, sitting, or standing), Walking, or Very Active. Separate scans are made for females and males, and simultaneous entries are also made for time of day, temperature, area accessibility, area usability, presence of supervision, presence and classification of organized activity, and equipment availability. Summary counts describe the number of males and females in any given setting and their activity levels. The instrument permits physical activity level comparisons to be made among different environments or within the same environment over different time periods. Energy expenditure rates (Kcal/kg/min) can also be calculated based on previously validated constants for each level of activity. (author abstract)