Ellipsometry has been applied to problems in the microelectronics industry from the beginning in the 1960's. More recently spectroscopic ellipsometry has been introduced. In-situ during process ellipsometry offers great promise for monitoring and control of a wide variety of microeletrcnics processes. This review covers some applications in silicon technology such as oxidation, chemical vapor deposition, etching, interfaces, and new processing techniques such as plasma, ion beam and rapid thermal, in an effort to demonstrate the kinds of crucial microelectronics information and processes that modern ellipsometry can access. The conclusion is that single wavelength ellipsometry alone is not sufficient; spectroscopic elilpsometry is required to establish the optimum ellipsometry measurement conditions. The future of ellipsometry in microelectronics is assessed