  The nuclear wave-function is dominated at low- and medium-x by gluons. As the
rapid growth of the gluon distribution towards low x, as derived from current
theoretical estimates, would violate unitarity, there must be a mechanism that
tames this explosive growth. This is most efficiently studied in colliders
running in e+A mode, as the nucleus is an efficient amplifier of saturation
effects occurring with high gluon densities. In fact, large A can lead to these
effects manifesting themselves at energies a few orders of magnitude lower than
in e+p collisions. In order to study these effects, there are proposals to
build an e+A machine in the USA, operating over a large range of masses and
energies. These studies will allow for an in-depth comparison to A+A collisions
where results have given tantalising hints of a new state of matter with
partonic degrees of freedom. In order to explain these results quantitively,
the gluons and their interactions must be understood fully as they are the
dominant source of hard probes at both RHIC and LHC energies.
