  The extreme extragalactic sources known as Ultraluminous X-ray Sources (ULX)
represent a unique testing environment for compact objects population studies
and the accretion process. Their nature has long been disputed. Their
luminosity, well above the Eddington luminosity for a stellar-mass black hole,
can imply the presence of an intermediate-mass black hole or a stellar black
hole accreting above the Eddington limit. Both these interpretations are
important to understand better the accretion process and the evolution of
massive black holes. The last few years have seen a dramatic improvement of our
knowledge of these sources. In particular, the super-Eddington interpretation
for the bulk of the ULX population has gained a strong consensus. Nonetheless,
exceptions to this general trend do exist, and in particular one ULX was shown
to be a neutron star, and another was shown to be a very likely IMBH candidate.
In this paper, I will review the progress done in the last few years.
