The use of physical activity monitors has increased dramatically in the past decade. Wrist-worn activity monitors are often used to monitor heart rate (HR) for exercise intensity. Assessing the validity of these devices is essential for accurate exercise prescription and user safety. PURPOSE: The objective of this study was to assess the validity of three common wrist-worn physical activity monitors for measuring HR during three modes of exercise. METHODS: Twenty-one participants (seven males, fourteen females) completed the study. Inclusion criteria for participation were: ≥ 18 years of age, have no history of cardiovascular or chronic disease (assessed by PAR-Q+), and have the ability to run for 10 minutes at 6mph. HR data was recorded for three common physical activity monitors (AM1, AM2, AM3) during 10 minutes of walking at 3mph, running at 6mph, and High Intensity Interval Training (HIIT) session consisting of a body weight series of exercises. HR data was collected every minute and compared against gold standard chest strap HR monitor to assess validity. Descriptive statistics were performed for all variables. R2 values were calculated for each device and each mode of exercise. RESULTS: AM1 was the most accurate device with R2 of 0.733 for running and 0.678 for walking. AM2 data showed moderate R2 for running (0.54) and walking (0.44). AM3 HR data was unreliable. HITT condition was the least accurate for each monitor (R2 range 0.098-0.124). CONCLUSION: These results demonstrate that the accuracy of the devices for tracking HR increases with increasing intensity of steady-state activities. HR data during HIIT was unreliable possibly due to excessive movements. Assessed physical activity monitors in this study are not accurate for tracking HR during walking, running, and HIIT particularly if used for patient monitoring. The cheapest monitor (AM3) had a very weak correlation; consumers should consider more expensive monitors for more accurate HR readings during steady-state activities. More research needs to be done to further explore the accuracy of activity monitors for a wider range of modalities and intensities.