Over the last two decades, there has been a proliferation of commercially available heart rate monitors. Recognizing that elite athletes often use heart rate to monitor training and assess aerobic fitness, fitness companies have offered a variety of heart rate monitoring systems to the general public. Recently, there has been a move from monitors that rely on chest straps to measure electrical activity toward more convenient, wrist-worn monitors that employ optical sensing technology similar to that used for pulse oximetry. While the accuracy of chest strap monitors has been assessed in a variety of studies, there is no data concerning the accuracy of wrist-worn heart rate monitors. Assessment of the monitors' accuracy is important both for the subjects who rely upon these monitors to guide their athletic activity and for the physicians to whom these individuals report their heart rate readings.