EUV observations could provide valuable and unique information about the nature of active galactic nuclei. I discuss their potential usefulness and review some of the main results obtained so far with the ROSAT WFC and the EUVE instruments. About three dozen sources, mostly Seyfert galaxies and BL Lacertae objects, have already been identified, and spectroscopic measurements of several of them have yielded a number of surprises. However, more observations are clearly needed to confirm some of the reported spectral features and to validate their interpretation.
The intraday variability in the radio-selected and
X-ray-selected BL Lacertae objects