  The study of the content, distribution and kinematics of interstellar gas is
a key to understand the origin and maintenance of both starburst and nuclear
(AGN) activity in galaxies. The processes involved in AGN fueling encompass a
wide range of scales, both spatial and temporal, which have to be studied.
Probing the gas flow from the outer disk down to the central engine of an AGN
host, requires the use of specific tracers of the interstellar medium adapted
to follow the change of phase of the gas as a function of radius. Current
mm-interferometers can provide a sharp view of the distribution and kinematics
of molecular gas in the circumnuclear disks of galaxies through extensive CO
line mapping. As such, CO maps are an essential tool to study AGN feeding
mechanisms in the local universe. This is the scientific driver of the NUclei
of GAlaxies (NUGA) survey, whose latest results are here reviewed. On the other
hand, the use of specific molecular tracers of the dense gas phase can probe
the feedback influence of activity on the chemistry and energy
balance/redistribution in the interstellar medium of nearby galaxies.
Millimeter interferometers are able to unveil the strong chemical
differentiation present in the molecular gas disks of nearby starbursts and
AGNs. Nearby active galaxies can be used as local templates to address the
study of more distant galaxies where both star formation and AGN activity are
deeply embedded.
