Published online by Cambridge University Press: 12 April 2016
Recent HST spectra indicate that a large fraction of Seyfert 1 galaxies have UV absorption lines that are intrinsic to the active nucleus or its environment. IUE monitoring or multiple HST observations of several sources show that many absorption lines are variable, either as a result of mass motion across the line-of-sight or of changing ionization within the gas in response to variations in the ionizing continuum. The UV absorbers may be related to the warm absorbers seen in the X-ray spectra of many Seyfert galaxies, but a direct connection between the two has not been made. Some of the absorption lines may arise in the emission-line regions in the nucleus or host galaxy, providing valuable clues to their nature.