X-ray fluorescence spectroscopy (XRF) is an elemental quantification analysis method for inorganic and metallic compounds. Sample preparation is simple and does not require solid samples to be dissolved, as is necessary for wet chemical analysis techniques. The fundamental parameter (FP) method implements quantitative analysis without type standards. The XRF technique has been widely accepted not only for bulk analysis but for thin film analysis in plating and electronic materials. With the thin film FP method, the thickness and composition of a thin film can be simultaneously determined, and the software is able to handle even complex multilayers. This article explains the basic principles of XRF thin film analysis; the depth of an XRF measurement and an overview of the thin film FP method. An ITO (indium tin oxide) thin film is analyzed as an example, and software parameter settings and reference material settings are also described.