Light Heavy Neutrinos

Heavy right-handed neutrinos are a natural addition to the particle content of the Standard Model. Within the high-scale seesaw mechanism they provide a natural explanation for the lightness of the active neutrinos. We here briefly discuss a few possible modifications of the standard seesaw scenario that permit relatively light heavy neutrinos at or below the electroweak scale. Such scenarios are experimentally testable and we touch upon the corresponding phenomenology at colliders and in meson decays.