Accelerometers are objective monitors that can be used to estimate energy expenditure (EE) during steady state exercise. However, high intensity interval training (HIIT) has received increasing attention because of its similar benefits to longer duration, steady state, less intense exercise. The accuracy of accelerometer-estimated EE during HIIT has yet to be examined. PURPOSE: The purpose of this study was to discern the differences between criterion-measured EE and accelerometer-estimated EE (kCals) during a HIIT session. METHODS: Nine participants (mean age=20.4 yrs, Body Mass Index=24.7 kg/m^2, males=8), completed a preliminary session, to determine treadmill speed at 95% HR max, and a HIIT session within 2 weeks of each other. For the HIIT session, each participant wore an ActiGraph GT3X+ accelerometer on their right hip while EE was measured using portable indirect calorimetry (Oxycon Mobile). The HIIT session comprised of 5 bouts: each bout included a 45-second exercise event and 90-second rest event. Data analysis was conducted using custom R scripts and paired T-tests to determine significant differences between criterion measure and accelerometer estimates of EE during the HIIT session. RESULTS: On average, the accelerometer underestimated total EE (92.76+0.33 kCals) compared with the criterion measure (108.73+3.99 kCals) for the entire HIIT session by 15% (p=0.0507). During exercise events, accelerometer estimated EE (8.99+1.99 kCals) was greater than criterion measured EE (7.10+1.82 kCals; p<0.001). During rest events, accelerometer estimated EE (9.56+2.86 kCals) was less than criterion measured EE (14.64+2.81; p<0.001). CONCLUSION: Compared with the criterion measure of indirect calorimetry, the accelerometer underestimated total EE for the HIIT session due to the underestimation of EE during rest events. Future studies should further investigate the accelerometer’s underestimation in larger more diverse samples to develop an algorithm that better predicts total EE during interval training.