interaction at low energies underwent a phase transition in the theoretical description with the introduction of Chiral Perturbation Theory ([ChPT]{}) in the early 1980s [@Weinberg:1978kz; @Gasser:1983yg; @Gasser:1984gg]. It allowed the theory of the lightest hadrons, the pions, kaons and eta, to be put on a solid theoretical footing. The main idea is that rather than a perturbative expansion in a small parameter like $\alpha$ or $\alpha_S$, there is a well-defined perturbation theory as an expansion in orders of momenta and masses. [ChPT]{} was also the prototype effective field theory, showing how to make sense of nonrenormalizable theories in a well-defined fashion.

The predictions of [ChPT]{} are of twofold type. There are the loop contributions at each order and the contributions that involve the parameters of the higher-order Lagrangians. This review summarizes the present knowledge of the values of these parameters. The standard name for these parameters is low-energy constants (LECs). The main part of this review concerns the LECs of two- and three-flavour mesonic [ChPT]{} in the isospin limit. Section \[sec:chpt\] gives an overview of the Lagrangians and serves to define our notation.

A first determination of the LECs was done in the papers where they were introduced [@Gasser:1983yg; @Gasser:1984gg]. 