  Precision measures of electroweak and flavour observables, at both low and
high energies, are highly complementary to direct searches for New Physics at
high-energy colliders. Despite the discovery of the Higgs boson at the Large
Hadron Collider, and of the overwhelming successes of the Standard Model,
several observational and theoretical problems remain to be addressed. In
addition to neutrino oscillation phenomena, the Standard Model fails to explain
the baryon asymmetry of the Universe, and does not offer a viable dark matter
candidate. In recent years, numerous deviations between the Standard Model
prediction and experimental measurements have been identified; interestingly,
most are closely connected to lepton flavours.
  In this thesis we have explored several aspects of flavour physics, focusing
on the phenomenological implications of models of massive neutrinos, and of
several Standard Model extensions capable of accommodating current tensions on
anomalous magnetic moments of charged leptons and several $B$-meson decay
observables.
