Two main physical processes characterize the activity in the nuclear
region of active galaxies: an intense star formation (starburst, SB)
and an Active Galactic Nucleus (AGN). While the existence of a
starburst-AGN connection is undisputed, still it is not clear which
process dominates the energetic output in both local and high redshift
Universe. Moreover there is no consensus on whether AGN fueling is
synchronous with star formation or follows it during a post-starburst
phase.  Here I first review the most recent techniques to disentangle
the relative SB-AGN contribution, then I focus on the physical and
geometrical properties of the circumnuclear environment.