In this talk, I will address various possible interactions between an active galactic nucleus (AGN) and the surrounding neutral hydrogen gas (HI), mainly using the HI 21-cm absorption technique.  Using results from GMRT and ASKAP surveys, I will discuss AGN-jet kinetics, the impact of AGN radiation on gas, and the redshift evolution of HI properties. I will present multiple new detections of HI 21-cm absorbers at extreme redshifts and AGN luminosity regimes, that will test the existing hypotheses related to AGN-associated gas. The detections are also first such, with the highest redshift, mass outflow rate, and optical depth, respectively.

I will discuss results from various VLBI instruments, about the parsec scale properties of multiple AGN-HI systems. These include aspects like gas covering factor against the radio source, spin temperature, gravitationally bound gaseous structures around the nucleus, and AGN jets constrained by dense gas. The discussion also includes pilot results from FLAVOS survey, which is a large follow-up VLBI survey for targets that have been detected with HI 21-cm absorption in the ongoing FLASH survey with ASKAP. 

I will also show our recent EVN+eMERLIN results on OH Megamaser emission at parsec scales, in the well-known system Arp220. Finally, I will talk about the prospects for the Square Kilometre Array (SKA).