SO Web-loquio: Old/new problems with Active Galactic Nuclei and AGN application to cosmology

After several decades of studies the basic nature of nuclear activity of galaxies is well understood. However, unexpected behaviour of AGN was already noted in the past, and with rise of the amount of data we see numerous evidences of phenomena which still require explanation, line Quasi-Periodic Ejection sources, and Changing-Look AGN. Also it is now time to address in more detail the physical nature of the simple AGN components like Broad Line Region and Dusty/Molecular torus. Despite these problems, the various correlations between the AGN parameters opens a way to use them also as tools in cosmology, although systematic errors in this case are likely underestimated.