2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/3135The properties of Compton-thick AGN are reviewed, with particular emphasis on BeppoSAX hard X-ray observations. I also discuss evidence for the presence of Compton-thick, circumnuclear matter in unobscured AGN, and I briefly introduce the HELLAS2XMM 5-10 keV survey, discussed in more details in other contributions to this volume.review talk at the conference "Issues in Unification of AGNs"Astrophysicstext