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). We studied the variability of the (50-100 keV)/(20-50 keV) hardness ratio derived from the light curves in 3 energy bands. The light curves are very well correlated, with a the hardest (50-100 keV) light curve variations slightly preceding the softer (20-50 keV) variations on time-scales of >= 1 d. NGC 4388, IC 4329 and NGC 4945 show spectral transitions in the form of a softening of the spectra with increasing flux. The results found indicate that the spectral state transitions observed are driven by changes of the optical depth of the corona, as previously suggested for the black hole binary system GX 339- -4.