This systematic review evaluated the validity and reliability of Fitbit and Jawbone activity trackers for measuring steps, distance, physical activity, energy expenditure, and sleep. A total of 22 studies were included, with 20 focusing on adults and 2 on youth. For laboratory-based step counting, both Fitbit and Jawbone showed high correlation (Pearson or intraclass correlation coefficients ≥ 0.80). Fitbit overestimated distance at slower speeds and underestimated at faster speeds, while Jawbone showed variable results. Energy expenditure was generally underestimated by both trackers. Sleep metrics were overestimated compared to polysomnography, with wake after sleep onset underestimated. No studies assessed intradevice reliability, but interdevice reliability was high for steps, distance, energy expenditure, and sleep for certain Fitbit models. The review found higher validity for steps, fewer studies on distance and physical activity, and lower validity for energy expenditure and sleep. The evidence indicated high interdevice reliability for steps, distance, energy expenditure, and sleep for certain Fitbit models. As new trackers are introduced, documenting their measurement properties is essential for research use. The review also highlighted the need for improved sleep measurement accuracy and more studies on distance and physical activity. Future research should focus on testing new trackers, exploring various activities, and considering privacy and informed consent issues. The study identified limitations, including potential missed studies and variability in methods. Overall, the review suggests that Fitbit trackers generally perform well for steps, distance, energy expenditure, and sleep, but further research is needed to confirm their reliability and validity.This systematic review evaluated the validity and reliability of Fitbit and Jawbone activity trackers for measuring steps, distance, physical activity, energy expenditure, and sleep. A total of 22 studies were included, with 20 focusing on adults and 2 on youth. For laboratory-based step counting, both Fitbit and Jawbone showed high correlation (Pearson or intraclass correlation coefficients ≥ 0.80). Fitbit overestimated distance at slower speeds and underestimated at faster speeds, while Jawbone showed variable results. Energy expenditure was generally underestimated by both trackers. Sleep metrics were overestimated compared to polysomnography, with wake after sleep onset underestimated. No studies assessed intradevice reliability, but interdevice reliability was high for steps, distance, energy expenditure, and sleep for certain Fitbit models. The review found higher validity for steps, fewer studies on distance and physical activity, and lower validity for energy expenditure and sleep. The evidence indicated high interdevice reliability for steps, distance, energy expenditure, and sleep for certain Fitbit models. As new trackers are introduced, documenting their measurement properties is essential for research use. The review also highlighted the need for improved sleep measurement accuracy and more studies on distance and physical activity. Future research should focus on testing new trackers, exploring various activities, and considering privacy and informed consent issues. The study identified limitations, including potential missed studies and variability in methods. Overall, the review suggests that Fitbit trackers generally perform well for steps, distance, energy expenditure, and sleep, but further research is needed to confirm their reliability and validity.