Spin can be a very difficult and confusing concept. Mathematically, spin can only be derived in relativistic quantum mechanics (i.e., quantum electrodynamics, QED). The word “relativistic” means that it applies to particles at high velocity, too. In QED, Dirac equation (a relativistic version form of Schrödinger equation) is used and the concept of spin of an electron appears naturally. In this book, we will treat spin as a given property of a particle. Indeed, in non-relativistic quantum mechanics, we take this for granted and we have been treating spin as an intrinsic property of a particle. However, its relationship to angular momentum and magnetic moment and its interaction with magnetic field need to be clarified to enhance our understanding and prepare us for more advanced studies in the future.