Ultra thin hard films such as diamond like carbon, carbon nitride or cubic boron nitride are instrumental for many high-tech products, e.g. to extend the useful life of tools or to protect the magnetic layers in hard disk drives. In the last years the development went to thinner protection layers. Especially carbon films with a thickness down to 2 nm were investigated using ellipsometry and x-ray reflectometry. Their thickness and optical constants were determined and correlations to other film parameters such as hardness, hydrogen or nitrogen content were found. The main problem of the optical characterization of these thin hard films on real functional layers is caused by the unknown optical constants of these underlayers. Thus these have to be measured simultaneously, which increases the number of unknown parameters significantly. As a consequence the development work for optical metrology of these products need more sophisticated methods like spectroscopic ellipsometry, Raman spectroscopy, x-ray reflectometry, and atomic force microscopy. With the data from these investigations in the development lab the real mass production can be controlled by simpler optical instrumentation such as single wavelength ellipsometry and FT-IR spectroscopy.