In the late 1970's, plasma etching emerged as a breakthrough method for pattern transfer in silicon integrated circuits. Soon after, it became apparent that, to further develop this technology, it was important to understand the mechanisms responsible for anisotropic plasma etching. Researchers in industry, national laboratories and universities began applying well established diagnostic techniques, and inventing new methods, for elucidating the chemical and physical processes underlying plasma etching, as well as plasma assisted deposition. Experiments were designed to either measure parameters of the plasma (species concentrations and velocities), or to simulate plasma-surface interactions under less complex conditions (high-vacuum / beam experiments, discharge flow tubes, etc). This talk will review the history of diagnostic methods for plasma processing. An admittedly incomplete survey of plasma diagnostic techniques will begin with several methods that greatly predate the microelectronics era. Electrical, optical and beam methods will be discussed and selected key experiments will be highlighted.