For those of the two-flavour or [$n_f=2$]{} case, the various LECs at next-to-leading order (NLO) can be determined in a rather straightforward fashion. The main analyses have been pushed to next-to-next-to-leading order (NNLO). This is reviewed in Sec. \[sec:nf2\] where we discuss the theoretical and experimental input to determine them.

The three-flavour or [$n_f=3$]{} coefficients were first determined in [@Gasser:1984gg] by using large-$N_c$ arguments, the relation with the [$n_f=2$]{} LECs, the pseudoscalar masses and $F_K/F_\pi$. The next step was to determine them from $K_{l4}$ decays at NLO [@Riggenbach:1990zp; @Bijnens:1989mr]. The first attempt at adding higher-order effects in determining the [$n_f=3$]{} LECs was Ref. [@Bijnens:1994ie]. The first full calculations at NNLO in [$n_f=3$]{} mesonic [ChPT]{}appeared in the late 1990s and a first fit using these expressions for the LECs was done in [@Amoros:2000mc; @Amoros:2001cp]. At this level, there was not sufficient information to really determine all LECs at NLO directly from data, in particular $L_4$ and $L_6$ are very difficult to obtain. The underlying reason for this is discussed in Sec. \[sec:largenc\]. Another difficulty is that quark masses and LECs cannot be disentangled without using more information [@Kaplan:1986ru]. We fix this ambiguity by using the quark mass ratio $m_s/\hat m$ as 