  There is considerable evidence that the circumnuclear regions of galaxies are
intimately related to their host galaxies, most directly through their bars.
There is also convincing evidence for relations between the properties of
supermassive black holes in the nuclei of galaxies and those of their host
galaxy. It is much less clear, however, how stellar (starburst) and non-stellar
(AGN) activity in the nuclear regions can be initiated and fuelled. I review
gas transport from the disk to the nuclear and circumnuclear regions of
galaxies, as well as the statistical relationships between the occurrence of
nuclear activity and mechanisms which can cause central gas concentration, in
particular bars and interactions. There are strong indications from theory and
modelling for bar-induced central gas concentration, accompanied by limited
observational evidence. Bars are related to activity, but this is only a weak
statistical effect in the case of Seyferts, whereas the relation is limited to
specific cases in starbursts. There is no observational evidence for a
statistical connection between interactions and activity in Seyferts, and some
evidence for this in starbursts, but probably limited to the extremes, e.g.,
ULIRGs. Some interesting hints at relations between rings, including nuclear
rings, and the presence of nuclear activity are emerging. It is likely that the
connection between the inflow of gaseous fuel from the disk of a galaxy on the
one hand and the activity in its nuclear region on the other is not as
straightforward as sometimes suggested, because the spatial- or time-scales
concerned may be significantly different.

Astrophysics