Massive black holes have been detected in many nearby galaxies. Their masses 
scale with several properties of the host galaxies, suggesting a correlation, 
and a coevolution, among black holes and galaxies. I will first discuss how 
gas accretion and black hole mergers shape this co-evolution at different 
galaxy and black hole masses. I'll then consider the relation (or lack 
thereof) between black hole accretion rate (BHAR) and the star formation rate 
(SFR) in the galaxy. In principle, the almost linear correlation between 
black holes mass and bulge/galaxy mass suggests parallel growth. 
Observationally, however, show different results on how BHAR compares to SFR: 
samples of AGN show no correlation, or at least a large scatter, between 
the mean BHAR and the mean SFR; samples of galaxies seem instead to find 
that they scale with each other. I will present the analysis of some 
simulations where the different aspects are analyzed, and seem to reconcile 
the different views.