  X-rays are a powerful probe of the physical conditions in the nuclei of
active galaxies. We review the X-ray properties of radio-quiet AGN, LINERs and
ultraluminous IR galaxies based on observations carried out with the X-ray
satellite ROSAT. We then summarize the observations of giant X-ray flares from
non-active galaxies, interpreted as stellar tidal disruptions by supermassive
black holes.
