Radio frequency (rf) plasmas are utilized in many applications in materials processing, such as semiconductor fabrication, surface modification, and coating. Plasma processing has replaced older techniques, such as wet chemistry, because the latter could not provide the necessary characteristics as process demands increased. A good example of this is the reduction of the feature size in semiconductors. The present critical dimension for semiconductor processing is 0.8 &mgr;m and is anticipated to be ≤0.25 &mgr;m by the year 2000. At present only plasma processing exhibits the potential of producing these line widths.An important factor, as the demands on the processing of materials become more critical, is exactly how to determine that the plasma is actually performing the process as designed. One way that is being investigated is to design control diagnostics that necessarily operate in real‐time,in situ, without significantly perturbing the process. Many such diagnostic methods have been proposed and are vigorously being investigated. They include probing the plasma with lasers, electric and mass selecting probes, and by observing the emission of radiation coming from the plasma. All of these and others must be investigated if the demands of material processing are to be met. Some of the methods being investigated for process control diagnostics are presented.