  This talk consists of four parts. In part one, I give an elementary
discussion on constructing a Lorentz-invariant spin sum rule for the nucleon.
In part two, I discuss a gauge-dependent spin sum rule, explore its relation
with the polarized gluon distribution, and introduce the complete evolution
equation for the spin structure. In part three, I consider a gauge-invariant
spin sum rule and the related evolution equation. The solution of the equation
motivates the possibility that half of the nucleon spin may be carried by
gluons at low energy scales. In the final part, I discuss deeply-virtual
Compton scattering as a possible way to measure the canonical orbital angular
momentum of quarks in the nucleon.
