Most if not all galaxies are thought to host a supermassive black hole in their nuclei. It grows by accreting mass, and while feeding it is not hidden from our view: it generates X-ray emission and ultraviolet that heats the dust which in turn radiates in the infrared. During the evolutionary phase in which it is most active, the object is known as an active galactic nucleus (AGN). The vast majority of AGN reside in normal galaxies in which the activity of star formation co-evolves with the black hole accretion, but astronomers disagree about the nature of the host galaxies, and in particular whether they resemble normal star forming galaxies in their overall structure.