  I discuss in this talk the physics of the $Q^2$ dependence of the
$G_1(x,Q^2)$ structure function sum rule. For $Q^2>3$ GeV$^2$, the $Q^2$
variation is controlled by pure QCD radiative corrections. For $0.5<Q^2<3$
GeV$^2$, the twist-four contribution becomes significant, but stays
perturbative. For $Q^2$ below $\sim 0.05$, the sum rule is determined by
low-energy theorems. The rapid change of the sum rule between 0.05 and 0.5
GeV$^2$ signals the transition between parton and hadron degrees of freedom.
