In recent years black holes of messes between a million and a billion solar masses have been detected and measured in the centers of many of galaxies, including our own, using dynamical methods such as star and gas motions. It has been noticed that the mass of the black hole is about 0.1% of the stellar mass in the central bulge of the host galaxy. A similar relation has been found in Seyfert galaxies and quasars, where the BH mass is derived by a very different technique, the reverberation mapping virial method. The study of such relations may lead us to understand how massive black holes are formed and grow in the centers of galaxies.