Add to ORKGChiral perturbation theory (ChPT) is an effective field theory that describes the properties of strongly-interacting systems at energies far below typical hadron masses. The degrees of freedom are hadrons instead of the underlying quarks and gluons. ChPT is a systematic and model-independent approximation method based on an expansion of amplitudes in terms of light-quark masses and momenta. The following is a brief overview of ChPT that is largely based on Scherer, Schindler, Lect. Notes Phys. 830 (2012), which can be referred to for a more detailed introduction.Comment: contribution to the review "50 Years of Quantum Chromodynamics," edited by F. Gross and E. Klempt [arXiv:2212.11107], to be published in EPJ