Accretion of matter by supermassive black holes residing at the centres of galaxies is responsible for triggering Active Galactic Nuclei (AGNs), the most energetic objects in the Universe. Observational and theoretical evidence collected in recent years strongly suggests that these objects play a crucial role in galaxy formation and evolution. They do this via the process of AGN feedback, where a fraction of the AGN energy output couples to the surrounding gas. AGN feedback regulates gas cooling, and has been invoked to explain the lack of a cooling catastrophe in galaxy clusters, and the dearth of widespread star formation in massive galaxies at low redshift. AGN can also enhance star formation; this mode of feedback may be important at high redshift.
I will review the evidence for AGN feedback, and discuss the different feedback modes. I will argue that understanding the processes that trigger AGN activity is crucial to quantifying feedback, and will outline how studying the complete "galaxy/environment - AGN triggering - feedback" cycle will help us understand the physics of this complex problem.