This book is designed as an advanced guide for experiments in Modern Physics. It covers the X Rays Fluorescence Experiment: theory, x-rays detectors, measurement of the photon energy, data analysis, determination of the energy calibration, interpretation of the lines seen in the energy spectra, the Moseley´s Law, the Statistical Methods: Covariance Ellipse and the Maximum Likelihood Method, etc. It is included the theory and the deduction of many formulas and the statistical analysis in order to clear the concepts and to get the concordance between the theory and the experiment. It is hoped that this book fill all needs of the students to get the fundaments of advanced experiments and to achieve the interest and motivation of the students for the real nature of the Physics.