We address the long standing problem of the construction of relativistic spin operators for a composite system in QCD. Exploiting the kinematical boost symmetry in light front theory, we show that transverse spin operators for massless particles can be introduced in an arbitrary reference frame, in analogy with those for massive particles. In light front QCD, the complete set of transverse spin operators are identified for the first time, which are responsible for the helicity flip of the

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... n. We establish the direct connection between transverse spin in light front QCD and transverse polarized deep inelastic scattering. We discuss the theoretical and phenomenological implications of our results.