Optical constants are specific properties of condensed matter that allow to describe in a simple way the interaction of light or other electromagnetic radiation with matter. There is always a requirement for optical constants values for estimating colour, reflection, internal total reflection, refraction, scattering, phase shifting, multilayer properties, or thin film thickness. This book is concerned with the practical determination of optical constants from easily accomplishable reflectance or transmittance measurements with reflectometers or ellipsometers. It provides information on the basics of optical constants and a comprehensive overview on models for optical constants. A brief overview on methods of measurement and evaluation is followed by guidelines to the practical determination of optical constants mainly of thin films on a substrate. The main task is always to find the best suited model for the parametrization of the optical constants.