2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/3304The evidence for obscured AGN and in particular for obscured quasars is discussed. The spectrum and source modelling of the X-ray Background suggests that most massive black holes grow by obscured accretion. A possible major growth phase which is Compton thick is explored and shown to be difficult to detect directly with current instruments.8 pages, 6 figures, presented at the meeting: "Issues in unifications of AGNs" Marciana Marina, May 2001; eds. R. Maiolino, A. Marconi and N. NagarAstrophysicstext