This lecture introduces X-Ray Fluorescence Spectrometry (XRFS), a chemical analysis technique that uses X-rays emitted by a material when excited by an X-ray primary beam. It covers X-ray sources, X-ray production, and the interaction between X-rays and matter. The lecture explains the principles of XRFS, including X-ray focusing, X-ray selection, and the excitation conditions. It also delves into the qualitative and quantitative analysis using XRFS, discussing the excitation-relaxation processes, X-ray emission spectrum, and the detection of X-rays. Additionally, it explores the errors, matrix effects, and sources of uncertainty in quantitative analysis by internal calibration and standardless methods.