During the past years, elliposometry, a non-destructive and contact-less optical surface analysis technique, has gained increased importance in industrial areas, such as the technology of electronic devices, when simple instruments, many of them computer-controlled and automated, became available. The potential users of such instruments are, however, frequently aware neither of the inherent possibilities of this technique, nor of its accuracy limitations. This book endeavors to point out some of the less obvious features and possibilities of ellipsometry, particularly of dynamic "in situ" measurements, and reviews its applications in research and manufacturing of semiconductor and thin film devices. A comprehensive discussion of various error effects typical particularly for simple ellipsometers and of their impact on measured sample parameters is provided. Error correction or (numerical) calibration procedures are given wherever possible, and design and operation guidelines for high-speed instruments suitable for dynamic "in situ" measurements are suggested.