  Simple dynamics, few available decay channels, and extremely well controlled
radiative and loop corrections, make pion and muon decays a sensitive means for
testing the underlying symmetries, the universality of weak fermion couplings,
as well as for study of pion structure and chiral dynamics. We review the
current state of experimental study of the allowed rare decays of charged
pions: (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 data
present an internally consistent picture that also agrees well with Standard
Model (SM) predictions. However, even following the great strides of the recent
decades, experimental accuracy is lagging far behind that of the theoretical
description for all above processes. We review the implications of the present
state of knowledge and prospects for further improvement in the near term.
