Published online by Cambridge University Press: 19 July 2016
Only little more than three years have passed since the launch of the Hubble Space Telescope (HST) and the wealth of results produced by astronomers using it, have already made fundamental contributions to our understanding of a variety of astrophysical processes. A considerable number of investigations have been, and are being, devoted to the study of the whole gamut of problems associated with activity in galaxies. These range from the very largest scales, namely those applicable to the study of the optical jets and galaxy mergers (10–100 kpc) to the smallest scales (1–10 pc) relevant to investigate the broad-line regions and the very center of the active galaxies. In all cases, the high-spatial resolutions, extended dynamic range and ultraviolet response, has made possible the study of a number of objects with a detail impossible without the HST.