X-ray fluorescence spectrometry (XRF) is a versatile tool in many analytical problems. Major, minor and trace elements can be qualitatively and quantitatively determined in various kinds of samples: metals, alloys, glasses, cements, minerals, rocks, ores, polymers as well as environmental and biological materials. Elements from Na to U are routinely determined using energy-dispersive X-ray fluorescence spectrometry (EDXRF) whereas application of wavelength-dispersive spectrometers (WDXRF) allows efficient determination of low-Z elements down to even Be. Although the samples can be analyzed without treatment, high quality results can be ensured if appropriate sample preparation is applied. This may vary from simple cleaning and polishing of the sample (metals, alloys), powdering and pelletizing with or without binder (ceramics, minerals, ores, soils, etc.), fusing the sample with appropriate flux (ceramics, rocks, ores, etc.) to digestion with acids (metals, alloys). This way errors resulting from surface roughness, particle size effect or inhomogeneity of the material can be eliminated or minimized. Due to the nondestructive character of X-ray measurement, the XRF spectrometry is widely applied in analysis of art, museum and archeological objects such as manuscripts, paintings, icons, pottery, ancient glasses, ceramics, coins. Moreover, XRF spectrometry is utilized for simultaneous determination of thickness and composition of various materials such as semiconductors, electrooptic and solar cell devices, etc., in electronic industry and other branches of technology. Typical detection limits for mediumand high-Z elements are in the ppm range, which is satisfactory for several applications. However, in some cases, the elemental concentrations are too low for a direct analysis. Then, the analytes must be preconcentrated prior to analysis using physical or chemical preconcentration or separation methods. Quantitative analysis of all types of aforementioned samples requires applying adequate empirical or theoretical methods. In quantitative XRF analysis, the measured fluorescent intensities are converted into the concentration of the analytes. This issue is rather complicated because the measured intensities depend not only on the analyte concentration but also on accompanying elements (matrix), sample type (solid, liquid or powder sample, etc.), method of sample preparation, shape and thickness of the analyzed sample and measurement conditions such as geometrical setup of the spectrometer, irradiated size, flux