"I will study the evolution of the masses and spins of massive black holes (MBHs) in competing scenarios for galaxy formation and for the MBH seeds at high redshift, and assess whether they may be discriminated with future gravitational-wave detectors and X-ray missions. This project is extremely timely and relevant for European science because two of the candidates (ATHENA and eLISA/NGO) for the ESA L-class mission selection for the Cosmic Vision program focus on MBHs as gravitational-wave or X-ray sources.The project will build upon some recent work of mine, in which I studied the MBH mass and spin evolution in a state-of-the-art ""standard"" semianalytical galaxy-formation model. To fully exploit ATHENA's and eLISA/NGO's potential, I will extend this work in several directions, namely:1) I will calculate the MBH merger rates for eLISA/NGO, in the ""standard"" galaxy-formation model but with different scenarios for the MBH seeds. I will include the effect of the spins on the gravitational waveforms using a novel effective-one-body model that I previously developed.2) I will consider an ""alternative"" galaxy-formation model that I proposed in 2010 and in which the baryonic evolution is driven by the two-phase structural evolution of the dark-matter halos, and I will work out its predictions for ATHENA and eLISA/NGO and the differences from the ""standard"" scenario.3) I will work out the preditions of both the ""standard"" and ""alternative"" galaxy-formation models for future gravitational-wave detectors in the 0.1-1 Hz frequency band (DECIGO and the Einstein Telescope).4) For both galaxy-formation models, I will analyze how well eLISA/NGO will measure the luminosity distance-redshift relation and therefore dark energy.My unique combination of expertise and the fact that I personally developed the tools needed for this project will be crucial to maximize scientific productivity and the impact on European science, and make me the ideal person for this research."