  We present an overview of searches fo violation of lepton flavor universality
with focus on low energy precision probes using pions, kaons, tau leptons, and
nuclear beta decays. The current experimental results are reviewed, the
theoretical status within the context of the Standard Model is summarized, and
future prospects (both experimental and theoretical) are discussed. We review
the implications of these measurements for physics beyond the Standard Model by
performing a global model-independent fit to modified $W$ couplings to leptons
and four-fermion operators. We also discuss new physics in the context of
simplified models and review Standard Model extensions with focus on those
which can explain a possible deviation from unitarity of the
Cabibbo-Kobayashi-Maskawa quark mixing matrix.
