What one can learn about QCD from nuclear beams at HERA

Overview is given of the theoretical issues of the physics which can be addressed with nuclear beams circulating in HERA. It is shown that such experiments widen considerably the horizon for probing QCD compared to that from free nucleon targets. They would allow to study nonlinear QCD phenomena at small x, understand dynamics of nuclear shadowing, as well as the the origin of diffraction in deep inelastic scattering. Interplay between the physics to be studied at HERA and in AA collisions at RHIC and LHC is also discussed.