Ellipsometry is a very powerful and totally nondestructive technique for determining optical constants, film thickness in multilayered systems, surface and interfacial roughness and material microstructure. Ellipsometric measurements can be made in vacuum, air and other environments. Ellipsometry has traditionally been used to determine film thickness and optical constants of dielectrics and optical coatings, semiconductors and heterostructures, magneto-optic, magnetic and opto-electronic materials, electrochemical, biological and medical systems and in surface modifications and surface roughness investigations. In situ measurements during crystal growth or material deposition are useful to study constituent fractions (including void fractions) in deposited or grown materials, surface oxide formation and film growth kinetics. Ellipsometric studies of metal, dielectric, semiconductor and organic layers carried out at the Institute of Physics of Wroclaw University of Technology by members of Thin Films Group are presented.