  Proton-nucleus (p+A) collisions have long been recognized as a crucial
component of the physics programme with nuclear beams at high energies, in
particular for their reference role to interpret and understand nucleus-nucleus
data as well as for their potential to elucidate the partonic structure of
matter at low parton fractional momenta (small-x). Here, we summarize the main
motivations that make a proton-nucleus run a decisive ingredient for a
successful heavy-ion programme at the Large Hadron Collider (LHC) and we
present unique scientific opportunities arising from these collisions. We also
review the status of ongoing discussions about operation plans for the p+A mode
at the LHC.
