Black holes are among the most mysterious objects in the Universe. Weighing up to several billion Suns, massive black holes have long been suspected to be the central powerhouses of energetic phenomena such as quasars. This book contains papers by some of the most influential astrophysicists working in this exciting field. They not only provide spectacular proof of the long-standing paradigm, but unexpectedly reveal that these objects, far from being rare, inhabit the center of virtually every large galaxy.