In brief: The validity of 13 commercially available heart rate (HR) monitors was assessed by comparing the monitored values with simultaneous ECG readings. Stability, as well as validity, was measured using several ergometric devices, and functionality was evaluated by analyzing the practical aspects of each device. Results indicate excellent correlations between readings obtained by ECG and HR monitors using conventional chest electrodes to measure electrical activity of the heart. Most of the monitors using other types of electrodes or using an earlobe photocell to measure opacity of blood flow were inadequate. The authors point out functional differences in the monitors to help potential users choose the best one to fit their needs.