  A future Electron-Ion Colllider (EIC) is the ideal laboratory for studying
the gluon distributions in both nucleons and nuclei for $\sqrt{s}$ = 63 - 158
(40 - 110) GeV/A for e+p (e+A) collisions. Whilst gluon distributions have been
studied extensively in nucleons at HERA, there is very little information on
them for $x <$ 0.1 in nuclei. The rapid increase in the gluon momentum
distribution at low-$x$ in nucleons found at HERA, if not tamed, leads to the
violation of the Froissart Unitarity Bound at small-$x$. This can be achieved
in non-linear QCD by allowing for the recombination of low-$x$ gluons until
saturation of gluon densities occurs. Understanding saturation is an important
goal not just for QCD in general, but also in understanding the initial
conditions of heavy-ion collisions in particular. In this paper, I describe the
physics that will be explored by e+A collisions, where saturation is more
easily explored in this mode at an EIC than in e+p collisions due to the large
nuclear densities achieved.
