The black holes of this chapter form, grow and evolve following the formation and assembly of dark matter halos and the collapse of their baryonic components during cosmic structure formation: we call them astrophysical black holes. These black holes weighing millions to ten billions of solar masses are sites of extreme and violent phenomena, and shape the environment with the energy they produce. Their existence is necessary to explain the emission from the population of Active Galactic Nuclei, the luminous flares of tidally disrupted stars in galactic nuclei, the fast motion of the S-stars at the Galactic Center and the long-lived jets emerging from M87-like galaxies. Supermassive black holes do not come to birth “supermassive” but must have grown from black hole seeds of intermediate mass \(\mathcal{O}(100-10^5\,\mathrm{M_\odot })\) . Here we describe avenues of seed formation at redshift \(z\lesssim 30\) , and investigate on their growth using “known” physics but extrapolated to untested astrophysical regimes and environments. The nature of seeds and their exceptionally fast evolution toward being massive is still unconstrained, and here we explore paths alternative to a primordial origin.