  Massive black holes are key inhabitants of the nuclei of galaxies. Moreover,
their astrophysical relevance has gained significant traction in recent years,
thanks especially to the amazing results that are being (or will be) delivered
by instruments such as the James Webb Space Telescope, Pulsar Timing Array
projects and LISA. In this Chapter, we aim to detail a broad set of aspects
related to the astrophysical nature of massive black holes embedded in galactic
nuclei, with a particular focus on recent and upcoming advances in the field.
In particular, we will address questions such as: What shapes the relations
connecting the mass of massive black holes with the properties of their host
galaxies? How do massive black holes form in the early Universe? What
mechanisms keep on feeding them so that they can attain very large masses at z
= 0? How do binaries composed of two massive black holes form and coalesce into
a single, larger black hole? Here we present these topics from a mainly
theoretical viewpoint and discuss how present and upcoming facilities may
enhance our understanding of massive black holes in the near future.
