  In this chapter, we introduce the concept of a black hole (BH) and recount
the initial theoretical predictions. We then review the possible types of BHs
in nature, from primordial, to stellar-mass, to supermassive BHs. Finally, we
focus on the latter category and on their intricate relation with their host
galaxies.

High Energy Astrophysical Phenomena;Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies;General Relativity and Quantum Cosmology