Measurements of g(1) at low x and low Q(2) are expected to provide a sensit
ive probe of the transition from Regge to perturbative QCD dynamics, offeri
ng a new testing ground for models of small x physics. We discuss the poten
tial of polarized ep colliders (Polarized HERA and eRHIC) to investigate th
is physics - varying Q(2) between 0.01 and 1 GeV2 - and to constrain the hi
gh-energy part of the Drell-Hearn-Gerasimov sum-rule for polarized photopro
duction.