  In this doctoral thesis, we study both low- and high-energy observables
related to massive neutrinos. Neutrino oscillations have provided indisputable
evidence in favour of non-zero neutrino masses and mixings. However, the
original formulation of the Standard Model cannot account for these
observations, which calls for the introduction of new Physics. Among many
possibilities, we focus here on the inverse seesaw, a neutrino mass generation
mechanism in which the Standard Model is extended with fermionic gauge
singlets. This model offers an attractive alternative to the usual seesaw
realisations since it can potentially have natural Yukawa couplings
($\mathcal{O}(1)$) while keeping the new Physics scale at energies within reach
of the LHC. Among the many possible effects, this scenario can lead to
deviations from lepton flavour universality. We have investigated these
signatures and found that the ratios $R_K$ and $R_\pi$ provide new, additional
constraints on the inverse seesaw. We have also considered the embedding of the
inverse seesaw in supersymmetric models. This leads to increased rates for
various lepton flavour violating processes, due to enhanced contributions from
penguin diagrams mediated by the Higgs and $Z^0$ bosons. Finally, we also found
that the new invisible decay channels associated with the sterile neutrinos
present in the supersymmetric inverse seesaw could significantly weaken the
constraints on the mass and couplings of a light CP-odd Higgs boson.
