One of the most significant continuing challenges in the phys- ical activity field is the need for valid and reliable measures of physical activity and sedentary behavior in adolescents for sur- veillance,epidemiological,andinterventionstudies.Concernover the validity of retrospective self-report due to recall errors and biases, especially for youth (1–3), has led to an increase in the use of “objective” measures of physical activity and sedentary behavior, such as accelerometer-based activity monitors, heart rate monitors, and global positioning system (GPS) devices. For instance, a number of studies have found differences in physi- cal activity levels and patterns when comparing self-report and objective (e.g., accelerometers and GPS) assessment methods (4– 6). Currently, objective activity monitors are being deployed