  We review the physics of purely leptonic decays of $\pi^\pm$, $K^\pm$,
$D^{\pm}$, $D_s^\pm$, and $B^\pm$ pseudoscalar mesons. The measured decay rates
are related to the product of the relevant weak-interaction-based CKM matrix
element of the constituent quarks and a strong interaction parameter related to
the overlap of the quark and antiquark wave-functions in the meson, called the
decay constant $f_P$. The interplay between theory and experiment is different
for each particle. Theoretical predictions of $f_B$ that are needed in the $B$
sector can be tested by measuring $f_{D^+}$ and $f_{D_s^+}$ in the charm
sector. The lighter $\pi^{\pm}$ and $K^{\pm}$ mesons provide stringent
comparisons between experiment and theory due to the accuracy of both the
measurements and the theoretical predictions. This review was prepared for the
Particle Data Group's 2012 edition.
