  Grand Unified Theories (GUTs) are one of the most interesting high-energy
completions of the Standard Model, because they provide a rich, powerful and
elegant group-theoretical framework able to resolve a variety of problems
remaining in our current understanding of particle physics. They usually act as
motivators for many low energy BSM theories, such as left-right symmetric or
supersymmetric models, and they serve to fill the gap between the
experimentally reachable low energies and the physics in the ultraviolet. In
recent years, however, they have fallen slightly from the spotlight, in favour
of `simplified' models with more specific phenomenological predictions. The aim
of this review is to summarize the state of the art on GUTs and argue for their
importance in modern physics. Recent advances in experiments permit to test the
predictions of GUTs at different energy scales. First, as GUTs can play a role
in the inflationary dynamics of the early Universe, their imprints could be
found in the CMB observations by the Planck satellite. Remarkably enough, GUTs
could manifest themselves also in terrestrial tests; several planned
experiments aim to probe the proton stability and to establish order of
magnitude higher bounds on its lifetime. Moreover, the predictions of specific
GUT models could be tested even at the LHC thanks to its high energy reach, via
searches for exotic states or additional contributions to flavour anomalies.
