  We review the status of the coupling constants of chiral Lagrangians in the
meson sector, the so-called low-energy constants (LECs). Special emphasis is
put on the chiral $SU(2)$ and $SU(3)$ Lagrangians for the strong interactions
of light mesons. The theoretical and experimental input for determining the
corresponding LECs is discussed. In the two-flavour sector, we review the
knowledge of the $O(p^4)$ LECs from both continuum fits and lattice QCD
analyses. For chiral $SU(3)$, NNLO effects play a much bigger role. Our main
new results are fits of the LECs $L_i$ both at NLO and NNLO, making extensive
use of the available knowledge of NNLO LECs. We compare our results with
available lattice determinations. Resonance saturation of LECs and the
convergence of chiral $SU(3)$ to NNLO are discussed. We also review the status
of predictions for the LECs of chiral Lagrangians with dynamical photons and
leptons.
