This book is designed as an advanced guide for experiments in Modern Physics. It covers the Cosmic Rays and Measurement of the Lifetime of Muons and Pions Experiment: cosmic rays, energy spectrum of the cosmic rays, ultra-hig energy cosmic rays, the GZK cutoff, the deflection angle of high energy cosmic rays in the propagation through the galactic and extragalactic magnetic flieds, the muon production and decay, scintillators and photomultipliers, determination of the mean lifetime, electronics logic, time calibration for muon and pions counting, spectrum of the muon and pion decay events, results with the uncertainties, etc. It is included the deduction of many formulas (as the GZK cutoff and the deflection angle of high energy cosmic rays) 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 fills 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.