Artificial insemination and embryo transfer programs are dependent on efficient and accurate detection of estrus. Visual observation is accurate at detecting animals in estrus, but efficiency ranges from approximately 50 to 70%. Electronic technologies have been developed in attempts to improve estrus detection efficiency. Commercially available electronic devices for estrus detection are based on changes in physical activity (pedometers), changes in electrical resistance of reproductive tract secretions (intravaginal resistance probes) or mounting activity (mount detectors). All of the commercially available electronic estrus detection devices can improve the efficiency of estrus detection in cattle. Pedometers are most applicable to lactating dairy cattle and have greater accuracy and efficiency when combined with visual observation. Intravaginal resistance measurement is perhaps the least practical method of estrus detection because of labor and animal handling requirements. Individual resistance measurement may have practical application for confirming other inconclusive signs of estrus. Mount monitors have the broadest application to beef and dairy cattle. HeatWatch, the only real-time radiotelometric system available, requires the least labor and animal handling and provides data on the time and duration of each mount. The less expensive stand-alone mount monitors also provide the necessary information for optimum timing of insemination and embryo transfer, but are more labor intensive.