  The Majorana nature of neutrinos is strongly motivated from the theoretical
and phenomenological point of view. A plethora of neutrino mass models, known
collectively as Seesaw models, exist that could generate both a viable neutrino
mass spectrum and mixing pattern. They can also lead to rich, new
phenomenology, including lepton number non-conservation as well as new
particles, that may be observable at collider experiments. It is therefore
vital to search for such new phenomena and the mass scale associated with
neutrino mass generation at high energy colliders. In this review, we consider
a number of representative Seesaw scenarios as phenomenological benchmarks,
including the characteristic Type I, II, and III Seesaw mechanisms, their
extensions and hybridizations, as well as radiative constructions. We present
new and updated predictions for analyses featuring lepton number violation and
expected coverage in the theory parameter space at current and future
colliders. We emphasize new production and decay channels, their
phenomenological relevance and treatment across different facilities in
$e^+e^-$, $e^-p$ and $pp$ collisions, as well as the available Monte Carlo
tools available for studying Seesaw partners in collider environments.
