Spin Dependent Structure Function $g_1$ at Low $x$
Theoretical expectations concerning small $x$ behaviour of the spin dependent structure function $g_1$ are summarised. This includes discussion of the Regge pole model predictions and of the double $ln^2(1/x)$ effects implied by perturbative QCD. The quantitative implementation of the latter is described within the unified scheme incorporating both Altarelli-Parisi evolution and the double $ln^2(1/x)$ resummation. The double $ln^2(1/x)$ effects are found to be important in the region of $x$ which can possibly be probed at HERA. Predictions for the polarized gluon distribution $\Delta G(x,Q^2)$ at low $x$ are also given.