This is not meant to be another review of ellipsometry, since a number of detailed works (1-5) have been published in recent years, including a very complete book by Azzam & Bashara (I). Rather we discuss why this technique, nearly a century old, has received so much recent attention (about 500 references for the period 1976-1979) and will probably continue to develop even faster. The method consists of measuring and interpreting th� change of polarization state that occurs when a polarized light beam is reflected at non-normal incidence from a sample surface. Its sensitivity lies in the submonolayer range, it is nondestructive, and it can be used in adverse ambient conditions including high temperature or high pressure, through gases or liquids, etc. Thus it appears perfectly adequate for tackling many of the problems of the semiconductor industry, which begins with well-polished reflecting substrates such as Si and GaAs and deposits on them thin films in the 1000 A range by various methods such as plasma reactions, chemical vapor deposition, and electrochemistry. Here lies the main reason for th�: growing interest in ellipsometry. The recent development of minicomputers and microprocessors provided the necessary hardware to allow widespread deVelopment of the method. It is now possible to build fully automatic instruments that can perform in real time both data-acquisition and data-reduction functions. Applications are discussed in two categories. Real-time analysis of thin film processing is presented first. In this case, measurements are made at a fixed wavelength and a fixed angle of incidence. The emphasis is on