Supermassive black holes lurk in the very centers of galaxies. Most galaxies have massive central black holes, in some cases weighing billions of solar masses. These were the sites of the most energetic phenomena in the Universe, that astronomers recognise as quasars. Such immensely luminous objects in the nuclei of galaxies were active when the universe was young. Even dwarf galaxies have central massive black holes. How did supermassive black holes form? Intermediate mass black holes may provide the missing link. The ultimate window on building black holes is gravity waves, and future experiments that willsearch for traces of their formation. I will conclude by presenting my views on primordial black holes. These are of great interest, since they are a promising, and possibly dominant, component of dark matter. I will discuss prospects for their detectability.