The environment surrounding accreting super-massive black holes is rich in 
gas and dust. Their structure is then a key ingredient in our understanding 
of nuclear activity, and holds clues to a number of astrophysical problems 
such as the AGN feeding from/feed-back onto the host galaxy, and the 
synthesis model of the Cosmic X-ray Background.
In this talk I will review recent results obtained with the hard X-ray 
(3-79 keV) focusing telescope NuSTAR in this field. These results allow us 
to probe the 3-D geometrical structure of the gas (covering and volume 
filling factors) through X-ray spectroscopy of heavily obscured AGN, as well 
its spatial scale through the structural changes (column density, ionisation) 
undergone by hot gas along the line-of-sight on time-scales as short as a few 
days. While the exact gas radial profile is still uncertain, a direct probe 
will be possible through high-resolution X-ray spectroscopy with ASTRO-H, 
the next high-energy mission of the Japanese Space Agency, scheduled to be 
launched by the end of this Japanese fiscal year.