Evidence has been accumulating for several decades that many galaxies harbour central mass concentrations that may be in the form of black holes with masses between a few million to a few billion times the mass of the sun.

Measurements over the last two decades, using adaptive optics imaging and spectroscopy on large ground-based telescopes, prove beyond any reasonable doubt the existence of such a massive black hole in the center of our own galaxy, the Milky Way.

These data also provide key insights into the black hole's properties and its environment. Future interferometric studies of the galactic center black hole promise to be able to test gravity in its strong field limit.

I will also briefly discuss the cosmological evolution of massive black holes.