  In this talk, I first motivate the use of Chiral Perturbation Theory in the
context of Lattice QCD. In particular, I explain how partially quenched QCD,
which has, in general, unequal valence- and sea-quark masses, can be used to
obtain real-world (i.e. unquenched) results for low-energy constants. In the
second part, I review how Chiral Perturbation Theory may be used to overcome
theoretical difficulties which afflict the computation of non-leptonic kaon
decay rates from Lattice QCD. I argue that it should be possible to determine
at least the O(p^2) weak low-energy constants reliably from numerical
computations of the K to pi and K to vacuum matrix elements of the
corresponding weak operators.
