Diagnostics and sensors are both measurement methods, but they have different connotations. Diagnostic equipment is used in the laboratory on research devices and therefore can be a large, expensive, and one-of-a-kind type of instrument. Sensors, on the other hand, are used in production and therefore have to be simple, small, unobtrusive, and foolproof. For instance, endpoint detectors, which signal the end of an etching step by detecting a spectral line characteristic of the underlying layer, are so important that they are continually being improved. Practical sensors are few in number but constitute a large subject which we cannot cover here. We limit the discussion to laboratory equipment used to measure plasma properties in processing tools.