Decay Constants of Charged Pseudoscalar Mesons
  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. 
Jonathan L. Rosner and Sheldon Stone