Ultraluminous X-ray Sources (ULXs) are, as suggested by their name, extremely luminous and rare X-ray emitting objects found in galaxies. Because of their luminosity, it has been suggested that they may be powered by accretion onto a black hole (BH) of a few 100 M⊙, more massive than what one would expect to originate from normal stellar evolution. Alternative models include young supernova remnants (SNRs) beamed emission from normal BH X-ray binaries (XRB) with high accretion rates, and relativistically beamed XRB omission. The observational evidence on ULXs suggests that while most of them are likely to be compact accreting objects, there is no clear unique evidence pointing either to the beamed XRB model or to accretion onto a very massive BH. It is possible that what we call ULXs are a heterogeneous family of X-ray sources.
