Purpose: This study aimed to establish physical activity (PA) intensity cutpoints for a wrist-mounted GENEActiv accelerometer (ACC) in elementary school-age children. A second purpose was to apply cutpoints to a free-living sample and examine the duration of PA based on continuous 1-s epochs. Methods: Metabolic and ACC data were collected during nine typical activities in 24 children age 6-11 yr. Measured V̇O2 values were divided by Schofield-estimated resting values to determine METs. ACC data were collected at 75 Hz, band pass filtered, and averaged over each 1-s interval. Receiver operator characteristic curves were used to establish cutpoints at sedentary (≤1.5 METs), light (1.6-2.99 METs), moderate (3.0-5.99 METs), and vigorous (≥6 METs) activities. These cutpoints were applied to a free-living independent data set to quantify the amount of moderate-vigorous PA (MVPA) and to examine how bout length (1, 2, 3, 5, 10, 15, and 60 s) affected the accumulation of MVPA. Results: Receiver operator characteristic yielded areas under the curve of 0.956, 0.946, and 0.940 for sedentary, moderate, and vigorous intensities, respectively. Cutpoints for sedentary, moderate, and vigorous intensities were 0.190g, 0.314g, and 0.998g, respectively. Intensity classification accuracies ranged from 27.6% (light) to 88.7% (vigorous) when cutpoints were applied to the calibration data. When applied to free-living data (n = 47 children age 6-11 yr), estimated daily MVPA was 308 min and decreased to 14.3 min when only including 1-min periods of continuous MVPA. Conclusions: Cutpoints that quantify movements associated with moderate-vigorous intensity, when applied to a laboratory protocol, result in large amounts of accumulated MVPA using the 1-s epoch compared to prior studies, highlighting the need for representative calibration activities and free-living validation of cutpoints and epoch length selection. © 2014 by the American College of Sports Medicine.