With the existence of supermassive black holes in galactic centers established beyond any reasonable doubt, the study of black hole demographics has become the focus of current research efforts. I will critically compare estimates of the mass density in supermassive black holes from the present day to the quasar era, derived using a variety of methods. These include dynamical mass estimates in local quiescent galaxies; reverberation mapping in active galaxies and quasars; and estimates of the mean density of compact objects based on quasar optical counts and observations of the X-ray background. I will then discuss how these observations can be used to learn about the formation and evolution not only of the black holes themselves, but also of their host stellar systems.