Supermassive black holes are nowadays believed to reside in most local galaxies. Accretion of gas and black-hole mergers play a fundamental role in determining the two parameters defining a black hole: mass and spin. I briefly review here some of the physical processes that are conducive to the evolution of the massive black-hole population. I'll discuss black-hole formation processes that are likely to place at early cosmic epochs, and how massive black holes evolve in a hierarchical universe. The mass of the black holes that we detect today in nearby galaxy has mostly been accumulated by accretion of gas. While black-hole–black-hole mergers do not contribute substantially to the final mass of massive black holes, they influence the occupancy of galaxy centers by black hole, owing to the chance of merging black holes being kicked from their dwellings due to the “gravitational recoil.” Similarly, accretion leaves a deeper imprint on the distribution of black-hole spins than black-hole mergers do. The differences in accretion histories for black holes hosted in elliptical or disk galaxies may reflect on different spin distributions.