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The origins of massive black holes

Massive black holes (MBHs) inhabit galaxy centers, power luminous quasars and Active Galactic Nuclei (AGN) and shape their cosmic environment with the energy they produce. The origins of MBHs remain a mystery and the recent detection by LIGO/Virgo of an almost 150 solar mass black hole has revitalized the question of whether there is a continuum between "stellar" and "massive" black holes and what the seeds of MBHs are. Seeds could have formed in the first galaxies, or could be also related to the collapse of horizon-sized regions in the early Universe. Understanding the origins of MBHs straddles fundamental physics, cosmology and astrophysics and it bridges the fields of gravitational wave physics and traditional astronomy. With several facilities in the next 10-15 years we foresee the possibility of discovering MBHs' avenues of formation. In this article we link three main topics: the channels of black hole seed formation, the journey from seeds to massive black holes, the diagnostics on the origins of MBHs. We highlight and critically discuss current unsolved problems and touch on recent developments that stirred the community.