  Supermassive black holes are nowadays believed to reside in most local
galaxies. Observations have revealed us vast information on the population of
local and distant black holes, but the detailed physical properties of these
dark massive objects are still to be proven. Accretion of gas and black hole
mergers play a fundamental role in determining the two parameters defining a
black hole: mass and spin. We briefly review here the basic properties of the
population of supermassive black holes, focusing on the still mysterious
formation of the first massive black holes, and their evolution from early
times to now.
