In an introduction, problems of modern thin film research and production of thin films are discussed. Possible solutions with different measurement methods like nulling ellipsometers, RAE-ellipsometry, spectroscopic ellipsometry, and interferometry are compared. RAE- ellipsometry with respect to precision for thin films below 100 angstrom are discussed. Repeatabilities of below 0.1 angstrom are shown. Thicker films even above a few microns can be evaluated with multi-wavelength measurements at distinct wavelengths at 543 nm, HeNe, 790 nm, IR 1.3 micrometers , and IR 1.5 micrometers . This interferometer emulation concept is practically free of order ambiguity--a problem with traditional ellipsometry. Multiple wavelengths are also used to characterize multilayers such as ONO, OPO, etc. with multiple results. Multiple wavelength ellipsometry is compared to multiple incidence angle ellipsometry. For multilayer stacks (transparent or absorbing) the SPI program was developed and is shown. In this program the known parameters as well as the to-be-measured parameters can be selected (substrate value, refractive index, k-value, thickness). Measurement spotsize effects are discussed--high lateral resolution measurements are presented.