In this talk I pretend to give a review of accretion onto black holes in different mass scales (black hole binaries, active galactic nuclei and ultra-luminous X-ray sources). A relativistic Fe K alpha line is present in the spectra of the most relevant sources from each class, e.g. in the black hole binary GX 339-4, in the Seyfert-1 galaxy 1H0707-495 and in a sample of the best ULX data from XMM-Newton. I will discuss implications from these results in the framework of the different models available. In the second part of the talk, I present the results of spectral variability in the 20-100 keV energy band from 58 months of continuous observations with the Swift/Burst Alert Telescope (BAT) of the 6 brightest AGNs in the hard X-ray band (NGC 4151, NGC 2110, NGC 4388, IC 4329, Circinus galaxy and NGC 4945). Eventually, I will present results obtained from the study of Ultra-Luminous X-ray sources and our prospects in the study of these intriguing sources.