Fluctuations in small systems

We review the main features of event-by-event fluctuations of the content of the Fock states of onia (as models for dilute hadrons, or as bare hadronic components of virtual photons), as well as some of their observable consequences. We briefly address the total scattering cross section of a small onium off a nucleus, then of two small onia. Finally, we explain that the multiplicity in the final state of collisions of large onia with nuclei may directly be related to the gluon density in the former. We provide first predictions for the event-by-event fluctuations of the gluon density.