The book reviews the present status of understanding the nature of the most luminous objects in the Universe, connected with supermassive black holes and supermassive stars, clusters of galaxies and ultraluminous galaxies, sources of gamma-ray bursts and relativistic jets. Leading experts give overviews of essential physical mechanisms involved, discuss formation and evolution of these objects as well as prospects for their use in cosmology, as probes of the intergalactic medium at high redshifts and as a tool to study the end of dark ages. The theoretical models are complemented by new exciting results from orbital and ground-based observatories such as Chandra, XMM-Newton, HST, SDSS, VLT, Keck, and many others.