So far, we have studied quantum mechanics as based on classical mechanics, relying on the correspondence principle. This approach makes it possible to obtain observable operators associated with measurable quantities such as energy or orbital angular momentum. The success of this method is a consequence of the fact that classical mechanics is a macroscopic limit of quantum mechanics. All macroscopic systems are made up of a large number of microscopic systems and their macroscopic properties keep a “memory” of certain properties at the microscopic level. The relationship between the microscopic and macroscopic domains is the subject of statistical physicsStatistical physics, where averages over large numbers of microscopic systems are taken. However, some microscopic properties can have macroscopic consequences that do not leave traces in classical mechanics. This is the case of the spinSpin of particles. The study of atoms, molecules and atomic nuclei is impossible without taking into account the notion of spin. The properties due to spin indirectly play a very large role in physics even if the macroscopic properties cannot reveal its existence.