Applications and limitations of optical spectroscopy for process control, as well as for more fundamental investigations, of glow discharge sputter deposition and emission spectroscopies allows the determination of concentrations and spatial distributions of sputtered and background gas species in ground, metastable, ionized, and excited states. Optical probing is a sensitive detection technique with good spatial resolution. It has the advantage of providing in situ real time information without disturbing the discharge. Examples of processes which may be monitored optically include: sputter etching and film deposition rates, net elemental sticking probabilities during bias sputter deposition, plasma ’’ashing’’ of photoresists, and reactive ion etching. Optical spectroscopy has also been used as a diagnostic technique for investigating preferential sputtering of alloys, reactive gas–surface interactions, and for providing quantitative chemical analysis and depth profiling information.