  In view of recent experimental indications of violations of Lepton Flavor
Universality (LFU) in $B$ decays, we analyze constraints and implications of
LFU interactions, both using an effective theory approach, and an explicit
dynamical model. We show that a simple dynamical model based on a $SU(2)_L$
triplet of massive vector bosons, coupled predominantly to third generation
fermions (both quarks and leptons), can significantly improve the description
of present data. In particular, the model decreases the tension between data
and SM predictions concerning: i) the breaking of $\tau$-$\mu$ universality in
$B\to D^{(*)} \ell \nu$ decays; ii) the breaking of $\mu$-$e$ universality in
$B \to K \ell^+\ell^-$ decays; iii) the difference between exclusive and
inclusive determinations of $|V_{cb}|$ and $|V_{ub}|$. The minimal version of
the model is in tension with ATLAS and CMS direct searches for the new massive
vectors (decaying into $\tau^+\tau^-$ pairs), but this tension can be decreased
with additional non-standard degrees of freedom. Further predictions of the
model both at low- and high-energies, in view of future high-statistics data,
are discussed.
