It was during the fifties that Academician Ambartsumian first raised the issue of the activity of nuclei of galaxies, long before the subject became fashionable. An elaboration on these ideas may be found for example in Ambartsumian's report to the 1970 Vatican meeting. The issues which he raised pertain to the physics of the nuclei and their interaction with the surrounding galaxy, and perhaps even beyond. In the meantime, a large body of data has become available and has been reported at this meeting, and it is clear that the activity of nuclei is of paramount importance, even though a common view on all aspects has not yet been achieved.