Abstract. The extreme emission of Ultraluminous X-ray Sources (ULX) represents 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, was often considered evidence for intermediate-mass black holes, or a signature of a poorly known regime of super-Eddington accretion. Both these interpretations are important to better understand 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, even more after the discovery of three neutron star-powered ULXs (the "mighty mice"), at 100-1000 times their Eddington luminosity.

I will review the progress done in the last few years on these exciting sources, and I will devote the second half to explaining some current efforts in the identification of their compact objects with a multi-wavelength study of variability.

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