  The RK measurement by LHCb suggests non-standard lepton-universality
violation (LUV) to occur in b to s l+ l- decays, with effects in muons rather
than electrons. It is intriguing that a number of other measurements of b to s
l+ l- transitions by LHCb and B-factories are consistent in magnitude and sign
with the RK effect, and fit a coherent effective-theory picture. Further
indications of non-standard LUV are provided by the long-standing discrepancies
in b to c tau nu transitions via the ratios R(D) and R(D^*). We review in
detail the experimental situation and its rich outlook, the theoretical efforts
-- and their challenges -- towards convincing dynamics beyond the
effective-theory level, and discuss the many directions of further
investigation that propagate from the current situation.
