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-time in situ etch 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.