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)xSU(2)xU(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."

The second edition of this monograph discusses the usefulness of heavy flavor as a probe of TeV-scale physics, exploring a number of recently-uncovered "flavor anomalies" that are suggestive of possible TeV-scale phenomena. The large human endeavor at the Large Hadron Collider has not turned up any New Physics, except the last particle of the Standard Model, the Higgs boson. Revised and updated throughout, this book puts the first results from the LHC into perspective and provides an outlook for a new era of flavor physics. The author readdresses many questions raised in the first edition and poses new ones. As before, the experimental perspective is taken, with a focus on processes, rather than theories or models, as a basis for exploration, and two-thirds of the book is concerned with b -^ s or bs sb transitions. In the face of the advent of Belle II and other flavor experiments, this book becomes a part of a dialogue between the energy/collider and intensity/flavor frontiers that will continue over the coming decade. Researchers with an interest in modern particle physics will find this book particularly valuable.