The Spin Structure of the Proton

It is shown that the proton "spin crisis'' or "spin puzzle" can be understood by the relativistic effect of quark transversal motions due to the Melosh-Wigner rotation. The quark helicity $\Delta q$ measured in polarized deep inelastic scattering is actually the quark spin in the infinite momentum frame or in the light-cone formalism, and it is different from the quark spin in the nucleon rest frame or in the quark model. The flavor asymmetry of the Melosh-Wigner effect for the valence $u$ and $d$ quarks and the intrinsic sea $q \bar{q}$ pairs are also the important ingredients in a SU(6) quark-spectator-diquark model framework to understand the "spin puzzle". Such a picture of the spin structure can be tested by use of several simple relations to measure the quark spin distributions in the quark model.