The massive nature of neutrinos provides a firm evidence for the existence of physics beyond the Standard Model, that in turn can give rise to a rich phenomenology in diverse experimental searches. We review the main features of the tree-level neutrino mass generation mechanisms, and, focusing on the extension of the Standard Model via the addition of Heavy Neutral Leptons, point to the specific phenomenology that is expected for minimal testable frameworks. We discuss the impact of an underlying lepton number symmetry on neutrinoless double beta decay, $\tau$ and meson decay and LHC searches.