The aim of this study was to determine thresholds for various intensities of physical activity in children and adolescents using the RT3 accelerometer. Forty healthy participants aged 10-16 years were recruited to the study. To validate the RT3 accelerometer data, an independent sample of 20 children and adolescents aged 10-16 years performed the same activities. Accelerometer data, heart rate, and oxygen consumption were measured at nine levels of physical activity, which varied in intensity: sedentary, light, moderate, and vigorous. Age and sex did not affect thresholds. The activity categories and accelerometer counts were: sedentary activity, 0-40 counts x min(-1); light activity, 41-950 counts x min(-1); moderate activity, 951-3410 counts x min(-1); and vigorous activity, >3410 counts x min(-1), respectively. These thresholds were considered valid as the difference between threshold values obtained using two independent groups of children was not significant. This study has established threshold values for various physical activities and enables the RT3 accelerometer to be used to quantify the duration of various levels of activity in adolescents under free-living conditions.