  This review has two principal aims. The first of these is to provide a
comprehensive overview of the applications of HNDs in the study of collections
of atoms and molecules, i.e. clusters and complexes. These clusters and
complexes must form through collisions inside HNDs, hence the title of this
review. A second aim is to provide a particularly detailed overview of the many
studies of ions, both positive and negative, that have been carried out in
HNDs.
