  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 nucleon. 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.
