The origin of the mysterious quantum spin of the electron and other charged ?particles? is shown to be a result of the Wave Structure of Matter (WSM). Spin is a spherical rotation in quantum space of the inward quantum waves of an electron at the electron wave-center to become the outward waves. Spherical rotation, a unique property of 3D space, can be described using SU(2) group theory algebra. The WSM spin agrees completely with the theoretical spin derived by Dirac (1929), and shows the physical origin of his Dirac Equation.