In the universe, we find 'light' black holes with masses similar to those of stars, and 'heavy' black holes with masses millions to billions of times the mass of a single star. These massive black holes grow within and with galaxies over billions of years. Their birth is enshrouded in the first, faraway galaxies. In this lecture, Volonteri will show us how we can model the formation and evolution of black holes with simulations and how we strive to unveil how they came into being, through observations combining light and gravitational waves.