  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.
