Spectroscopic ellipsometry (SE) is extensively used for thin-film-growth monitoring. SE can provide a lot of information about thin films, such as the optical gap, electronic structure, physical structure, composition, and stoichiometry, as well as their density and thickness. Moreover, SE analysis can be extended to multilayer systems that can be used in situ to identify the various stages of ... [Show full abstract] the film growth, and it is very sensitive to the surface and bulk properties of the materials. The chapter provides an overview of the effective-medium theory and microscopic surface roughness to understand and quantify the information about the properties and growth stages of thin films during and after preparation. Among the various ellipsometric techniques available, rotating analyzer ellipsometry (RAE) at low frequency and phase-modulated spectroscopic ellipsometry (PMSE) at high frequency are the most important ones. The interaction between a material and a polarized light beam, incident at an angle ө and reflected from the material's surface, modifies the polarization of the beam.