  The existing Chiral Perturbation Theory (ChPT) calculations at order $p^6$
are reviewed. The principles of ChPT and how they are used are introduced. The
main part is a review of the two- and three-flavour full two-loop calculations
and their comparison with experiment. We restrict the discussion to the mesonic
purely strong and semileptonic sector. The review concludes by mentioning the
existing results in finite volume, finite temperature and partially quenched
ChPT.
