Ultra-Luminous X-ray Sources (ULX) are off-nuclear X-ray sources in nearby galaxies whose luminosities exceed $10^{39}$ ergs/s. Their high luminosity implies that, if they are sub-Eddington sources, they harbour black holes in the intermediate mass range of 100 to 10000 solar masses. Alternatively, they could be stellar mass black holes radiating at super-Eddington luminosities which would imply a new type of accretion process not directly observed before. The speaker will present some recent analysis of the X-ray properties of ULX and identification of optical counterparts. He will discuss the implications of the results to our understanding of the accretion disk geometry and nature of these enigmatic sources.