Abstract: This study highlights the recent research progress of an online yield monitoring system for grain combine harvesters in modern agriculture. The current measurements for the online yield monitoring were also summarized into the dynamic weighing, volumetric, impulse, and nuclear radiation measurement. Specifically, the dynamic weighing measurement was used for the weighing sensors to weigh the grain bins, screw conveyors, and elevators. The volumetric measurement mainly used the through-beam photoelectric sensors, diffuse reflection photoelectric sensors, and machine vision to measure the volume of grain on the scraper. More importantly, the volume sensor of the scraper wheel was selected to obtain the flow and volume of grain. The impact-based sensors were also adopted to measure the grain flow at the outlet of the elevator. Two kinds of sensors were included, the piezoelectric and strain impact-based grain flow sensors. Nuclear radiation measurement mostly used gamma and X-ray sensors to measure the grain flow. Other measurements were involved to establish the relationship between machine operating parameters and grain quality, such as the torque, tension, current, and speed sensors. At the same time, the limitations of each measurement were evaluated, in terms of stability and accuracy, feasibility, and versatility during the operation of the grain combine harvester. Furthermore, the grains were also dynamically weighed through the grain tank and the scraper due to inertia under specific conditions, such as the machine vibration, the bumps caused by the uneven ground in the field, the tilting, or the turning of ground, and the emergency brake. Nevertheless, the quality and volume were prone to drastic changes in the grain bins or scrapers using the grain volume measurement. Particularly, the accuracy of yield measurement was relatively low, due mainly to the influence of crop varieties, density, and moisture content. Since a large installation space was required for some yield monitoring, such as the auger weighing of net grain in a screw conveyor, capacitance measurement, and working parameters to determine the yield, the compact design of a combine harvester brought the complicated and labor-intensive modification for the installation of measurement system under the limited room. In addition, the low applicability of nuclear radiation was attributed to the possible radiation hazards for the human body during the production test. At the same time, the mechanical damage to the grain was inevitably caused by the collision of grain and impact plate during the impact measurement. As such, the quality and grade of processed grains were reduced for the germination rate of seed, due to the damaged grain was more prone to mildew during storage. Correspondingly, the yield measurement, sensor installation, and location varied significantly in the different physical and chemical properties of various harvested crops, as well as the different net grain transportation structures. Some suggestions were also proposed for crop yield monitoring during this time. Anyway, the simplest and most convenient installation was preferred to meet the needs of different crop harvesting, such as the integrated, miniaturized, high-precision, and intelligent measurement sensors with the communication interface standard. Consequently, an intelligent system can be expected to develop in the combine harvesters for real-time monitoring, such as crop output, harvester geographic location, and output distribution map.