  Measurements of g_1 at low x and low Q^2 are expected to provide a sensitive
probe of the transition from Regge to perturbative QCD dynamics, offering a new
testing ground for models of small x physics. We discuss the potential of
polarized ep colliders (Polarized HERA and eRHIC) to investigate this physics
--- varying Q^2 between 0.01 and 1 GeV^2 --- and to constrain the high-energy
part of the Drell-Hearn-Gerasimov sum-rule for polarized photoproduction.
