Supermassive black holes and AGN co-evolve over cosmic time, but despite more than a decade of research the engine(s) for this co-evolution are still not fully understood. The typical co-evolution picture invokes major galaxy mergers to both drive material toward the centre of the gravitational potential and trigger star formation, growing both galaxy and black hole together. This is certainly a plausible explanation for many observed systems, but there is growing evidence from both local ground-based and higher-redshift HST observations that supermassive black holes often grow in systems that cannot have had a major merger. This talk will review the field of black hole-galaxy co-evolution from 0 < z < 3 and across many orders of magnitude in the AGN luminosity and black hole mass function, and discuss the relative importance of both mergers and completely calm, “secular” evolution on black hole growth. New evidence from recent years suggests merger-free process may contribute significantly to both the overall growth of supermassive black holes and their co-evolution with their host galaxies.