  In the framework of renormalisable relativistic quantum field theory, the
explanation of neutrino masses necessarily requires the existence of new
physical states. These new states may also be responsible for other unexplained
phenomena in particle physics and cosmology. After a brief introduction, I
focus on scenarios in which the neutrino masses are generated by the type-I
seesaw mechanism and review the phenomenological implications of different
choices of the seesaw scale.
