The ?avor sector carries the largest number of parameters in the
Standard Model of particle physics. With no evident symmetry principle
behind its existence, it is not as well understood as the
SU(3)×SU(2)×U(1) gauge interactions. Yet it tends to be underrated,
sometimes even ignored, by the erudite. This is especially so on the
verge of the LHC era, where the exploration of the physics of
electroweak symmetry breaking at the high energy frontier would soon be
the main thrust of the ?eld. Yet, the question of "Who ordered the
muon?" by I. I. Rabi lingers. We do not understand why there is "family"
(or generation) replication. That three generations are needed to have
CP violation is a partial answer. We do not understand why there are
only three generations, but Nature insists on (just about) only three
active neutrinos. But then the CP violation with three generations fall
far short of what is needed to generate the baryon asymmetry of the
Universe. We do not understand why most fermions are so light on the
weak symmetry breaking scale (v. e. v. ), yet the third-generation top
quark is a v. e. v. scale particle. We do not understand why quarks and
leptons look so different, in particular, why neutrinos are rather close
to being massless, but then have (at least two) near maximal mixing
angles. We shall not, however, concern ourselves with the neutrino
sector. It has a life of its own.