2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/5027Little is presently known about the nature of ultraluminous X-ray sources (ULX). Different hypotheses have been proposed to explain their properties: intermediate-mass BHs, Kerr BHs, young SNR, or background AGN. Some of the current problems and open questions in this research field are here reviewed.4 pages, no figures. Presented at AGN05: Inflows, Outflows and Reprocessing around black holes, Como (Italy), 11-14 June 2002. To appear in the ProceedingsAstrophysicstext