Demands for increased productivity and improved quality have been the impetus for realtime process monitoring and control. While a process monitor can be used to compare the output of the sensor to predefined parameter limits, process control goes a step beyond process monitoring by adjusting the welding process automatically. This chapter from the Welding Handbook, 9th Ed., Vol. 1, discusses principles behind process sensing, monitoring, and control and describes the application of sensors, monitors, and process controls in sections covering the arc, resistance, laser beam, electron beam, friction welding, and brazing processes.