Purpose: Wrist-worn monitors are developed to unobtrusively measure heart rate (HR) at rest and during exercise. assessed the concurrent validity and reliability of the Microsoft Band 2 (Microsoft-Band2) and Fitbit Charge HR (Fitbit) to measure HR at rest and during exercise. Methods: Healthy men (n=12) and women (n=12) (mean (± SD); age 24.3 ± 3.1 yr) were tested on two occasions separated by at least 7 d. The same protocol was used during each visit and consisted of 3-min conditions in the following order - supine, sitting, 6 km.h-1 walk, 10 km.h-1 run, and 12.km.h-1 run. HR was continuously measured using a Holter monitor, Microsoft-Band2, and Fitbit, and averaged across each 3-min condition. A Bland Altman analysis was conducted to calculate the intervals of agreement (95%). A 2 tailed t-test at α = 0.05 was also used to compare the mean differences in measurements with the Holter for both devices and an F-test (α = 0.05) was used to compare the measurement dispersion characteristics of both devices. Results: The intervals of agreement for the Fitbit had comparable dispersion characteristics with the Microsoft-Band2 with the exception of the supine condition (p = 0.004). The difference between Fitbit and Holter are significantly further from zero than the difference between Microsoft-Band2 and Holter for sitting (p = 0.004) and 6 km.h-1-walk (p = 0.001). Conclusion: Microsoft-Band2 is more accurate than Fitbit at seated rest and during low intensity exercise, walking, and is comparable to Fitbit at 10km.h-1 run.