This book is designed as an advanced guide of theory for experiments in Modern Physics. It covers the theory of the next Modern Physics fields: X Rays, Compton Scattering, Beta decay, Cosmic Rays, Muon Decay, Nuclear Decay, Gamma Rays, X Rays Diffraction. It is included the deduction of many formulas in order to clear the concepts and to fill all needs of the students to get the fundaments of advanced experiments. Finally, other goal of the book is to achieve the interest and motivation of the students for the real nature of the Physics.