First, an introductory review of deposition dependent microstructural peculiarities of real optical thin films is presented. Then, some physical and chemical properties of particular interest for the user of coated optical elements are surveyed. Of the optical properties, the refractive index is discussed in detail, and reflectivity, light scattering and surface plasmon excitation are also included. Of the mechanical properties, adhesion, hardness, abrasion as well as some aspects of intrinsic stress are illustrated with practical examples. The importance of the thermal stability of optical coatings is emphasized, and some examples of severe alterations of coating properties after thermal treatment are given. The discussion of diffusion processes within some coatings and in the surface region of substrates leads to a survey of selected chemical reactions between the substrate and the coating, within the coating itself and between the coating and the environment. Finally, the degradation of coatings as a result of radiation impact is discussed shortly.