Black holes are among the most mysterious objects that the human mind has been capable of imagining. As pure mathematical constructions, they are tools for exploiting the fundamental laws of physics. As astronomical sources, they are part of our cosmic landscape, warping space-time, coupled to the large-scale properties and life cycle of their host galaxy, and perhaps even linked to galaxy formation. This volume, which grew from a recent doctoral school sponsored by the Italian Society of Relativity and Gravitation, brings together contributions from leading authorities to provide a review of recent developments in the study of the astrophysical black holes that inhabit nearby galaxies and distant quasars. These lectures reveal the deep symbiotic relationship between black holes and their cosmic environment and show that black holes are key sources for exploring not only our local universe, but also our cosmic dawn. Topics range from the observational evidence for supermassive black holes and the joint evolution of black holes and galaxies to the cold dark matter paradigm of hierarchical galaxy formation and from the cosmic history of the diffuse intergalactic medium to the ecology of black holes in star clusters.