Spectroscopic ellipsometry (SE) has proven to be a very powerful diagnostic for thin film characterization, but the results of SE experiments must first be compared with calculations to determine thin film parameters such as film thickness and optical functions. This process requires four steps. (1) The quantities measured must be specified and the equivalent calculated parameters identified. (2) The film structure must be modeled, where the number of films is specified and certain characteristics of each layer specified, such as whether or not the film is isotropic or anisotropic, homogeneous or graded. (3) The optical functions of each layer must be specified or parameterized. (4) The data must be compared with the calculated spectra, where a quantifiable figure of merit is used for the comparison. The last step is particularly important because without it, no "goodness of fit" parameter is calculated and one does not know whether or not the calculated spectrum fits the data.