I. INTRODUCTION The term “ flavors ” is used, in the jargon of particle physics, to describe several copies of the same gauge representation, namely several fields that are assigned the same quantum charges. Within the Standard Model (SM), when thinking of its unbroken SU (3)$_{C}$ × U (1)$_{EM}$ gauge group, there are four different types of particles, each coming in three flavors: The term “ flavor physics ” refers to interactions that distinguish between flavors. By definition, gauge interactions, namely interactions that are related to unbroken symmetries and mediated therefore by massless gauge bosons, do not distinguish among the flavors and do not constitute part of flavor physics. Within the Standard Model, flavor-physics refers to the weak and Yukawa interactions. With New Physics (NP), there are likely to be additional ‘flavored’ interactions. The term “ flavor changing ” refers to processes where the initial and final flavor-numbers (that is, the number of particles of a certain flavor minus the number of anti-particles of the same flavor) are different. In “flavor changing charged current” processes, both up-type and down-type flavors, and/or both charged lepton and neutrino flavors are involved. Examples are (i) muon decay via µ → e ¯ ν$_{i}$ν$_{j}$ , and (ii) K − → µ $^{−}$¯ ν$_{j}$ (which corresponds, at the quark level, to s ¯ u → µ $^{−}$¯ ν$_{j}$ ). Within the Standard Model, these processes are mediated by the W -bosons and occur at tree level. In “ flavor changing neutral current ” (FCNC) processes, either up-type or down-type flavors but not both, and/or either charged lepton or neutrino flavors but not both, are involved. Example are (i) muon decay via µ → eγ and (ii) K$_{L}$ → µ $^{+}$µ − (which corresponds, at the quark level, to s ¯ d → µ $^{+}$µ $^{−}$). Within the Standard Model, these processes do not occur at tree level, and are often highly suppressed. This situation makes FCNC particularly sensitive to new physics: If the new physics does not have the same flavor suppression factors as the standard model, then it could contribute to FCNC comparably to the standard model even if it takes places at energy scales that are orders of magnitude higher than the weak scale. The fact that flavor physics is a very sensitive probe of high energy physics is the main reason for the experimental effort to measure flavor parameters and the theoretical effort to interpret these data. 2 • Up-type quarks in the (3)$_{+2}$$_{/}$$_{3}$ representation: u, c, t ; • Down-type quarks in the (3)$_{−}$$_{1}$$_{/}$$_{3}$ representation: d, s, b ; • Charged leptons in the (1)$_{−}$$_{1}$ representation: e, µ, τ ; • Neutrinos in the (1)$_{0}$ representation: ν$_{1}$, ν$_{2}$, ν$_{3}$ .