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Simple dynamics, few available decay channels, and extremely well controlledradiative and loop corrections, make pion and muon decays a sensitive means fortesting the underlying symmetries, the universality of weak fermion couplings,as well as for study of pion structure and chiral dynamics. We review thecurrent state of experimental study of the allowed rare decays of chargedpions: (a) electronic, $\pi^+ \to e^+\nu_e$, or $\pi_{e2}$, (b) radiative,$\pi^+ \to e^+\nu_e\gamma$, or $\pi_{e2\gamma}$, and (c) semileptonic,$\pi^+\to \pi^0 e^+ \nu$, or $\pi_{e3}$, as well as muon radiative decay,$\mu^+\to e^+ \nu_{\text{e}}\bar{\nu}_{\mu}\gamma$. Taken together, these datapresent an internally consistent picture that also agrees well with StandardModel (SM) predictions. However, even following the great strides of the recentdecades, experimental accuracy is lagging far behind that of the theoreticaldescription for all above processes. We review the implications of the presentstate of knowledge and prospects for further improvement in the near term.

Allowed rare pion and muon decays as tests of the Standard Model