Observations indicate the existence of Quasi Stellar Objects (QSOs) at high redshift powered by supermassive black holes. This raises different questions about the mechanism(s) required to form these objects in timescales less than 1 Gyr. I will show that there are two possible evolutionary paths: a first one, in which seeds of about 100 solar masses grow intermittently producing the large majority of supermassive black holes seen in the local universe and a second one, in which those seeds grow in a unique accretion episode provided by a massive accretion disk. Results derived by simulations will be shown.