During the course of the Microelectronics Manufacturing Science and Technology (MMST) program, a number of sensors have been integrated into various test‐bed plasma etch reactors with the goal of monitoring, diagnosing, and controlling these processes. These sensors include single wavelength and spectral ellipsometers for real‐timeinsituetch rate and endpoint determination; a standard monochromator for etch rate and nonuniformity measurements; an eddy current sensor for incoming metal thickness control; a rf monitor for rf current and voltage diagnostics; and a scatterometry‐based critical dimension sensor for linewidth measurements. The full integration of these sensors turned out to be a complex and time‐consuming task including hardware, optical, software, material, and processing issues. Once integrated into the reactor, and the appropriate process modeling completed, these sensors enabled the monitoring, diagnosis, and model‐based control of these processes. Besides maintaining specific process observables at their target values, running under process control has highlighted phenomena such as equipment aging and first‐wafer effects. This work has clearly shown that the full implementation of sensors into commercial manufacturing equipment is essential for the model‐based control and diagnosis of semiconductor processes.