  We review here the physics of purely leptonic decays of pi-, K-, D+, Ds+, and
B- 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 anti-quark wave-functions in the meson, called the decay constant fP. The
interplay between theory and experiment is different for each particle.
Theoretical predictions that are necessary in the B sector can be tested, for
example, in the charm sector. One such measurement, that of fDs, differs from
the most precise unquenched lattice calculation and may indicate the presence
of new intermediate particles, or the theoretical prediction could be
misleading. The lighter pi and K mesons provide stringent comparisons due to
the accuracy of both the measurements and the theoretical predictions. This
review was prepared for the Particle Data Group's 2008 edition.
