Proof of neutrino masses and mixing from a plethora of
experiments have opened a window to physics beyond the standard model
of particle physics. There are well-defined underlying theories which
explain the electron-volt neutrino masses. It is well known that
Seesaw mechanism has its success in explaining the smallness of neutrino
mass. The mechanism predicts Majorana mass of the light neutrinos, which
violate lepton number symmetry of the Standard Model. In this talk, I
will discuss about one of the most common ingredient of neutrino mass
generation, which is the presence of heavy neutral lepton states in the
model, and their related phenomenology, and collider and non-collider
probes.
