Every massive galaxy is believed to host a supermassive black hole (SMBH) at its center. Its mass is correlated with the mass of the inner regions of its host (and also with some other properties), probably because the SMBH grows and evolves as the galaxy itself grows, through mergers with other galaxies and the infall of material from the intergalactic medium. When material makes its way to the galactic center and accretes onto the SMBH, it produces an active galactic nucleus (AGN); outflows or other feedback from the AGN then act disruptively to quench star formation in the galaxy. Modern cosmological simulations now self-consistently trace star formation and SMBH growth in galaxies from the early universe to the present day, confirming these ideas.